Fix L49/L53/L59: error string newlines, dialog null-safety, shared theme constant

L49: Remove newline from missing-binary error message in download.ts; inline
     error spans render it as literal newline, now one sentence with period.
L53: backup.ts exportBackup/importBackup/showMessageBox no longer use win!
     non-null assertions -- each uses a guarded ternary (same pattern index.ts
     already used for chooseFolder).
L59: Extract PAGE_BACKGROUND to src/shared/theme.ts; main/index.ts and
     renderer/src/theme.ts both import from it -- no more "keep in sync" comment.

typecheck + 242 tests + eslint + prettier green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/backup.ts

@@ -12,10 +12,11 @@ interface BackupFile {
 }
 
 export async function exportBackup(win: BrowserWindow | undefined): Promise<BackupExportResult> {
-  const res = await dialog.showSaveDialog(win!, {
+  const opts = {
     defaultPath: 'aerofetch-backup.json',
     filters: [{ name: 'JSON', extensions: ['json'] }]
-  })
+  }
+  const res = await (win ? dialog.showSaveDialog(win, opts) : dialog.showSaveDialog(opts))
   if (res.canceled || !res.filePath) return { ok: false }
   // Strip credentials (proxy creds, API tokens) so a backup file shared or synced
   // to the cloud doesn't leak secrets in plaintext (M22). The user re-enters them
@@ -32,10 +33,11 @@ export async function exportBackup(win: BrowserWindow | undefined): Promise<Back
 }
 
 export async function importBackup(win: BrowserWindow | undefined): Promise<BackupImportResult> {
-  const res = await dialog.showOpenDialog(win!, {
-    properties: ['openFile'],
+  const openOpts = {
+    properties: ['openFile' as const],
     filters: [{ name: 'JSON', extensions: ['json'] }]
-  })
+  }
+  const res = await (win ? dialog.showOpenDialog(win, openOpts) : dialog.showOpenDialog(openOpts))
   if (res.canceled || !res.filePaths[0]) return { ok: false }
 
   let parsed: unknown
@@ -86,8 +88,8 @@ export async function importBackup(win: BrowserWindow | undefined): Promise<Back
       })
       .join('\n')
     const more = commandTemplates.length > 10 ? `\n…and ${commandTemplates.length - 10} more.` : ''
-    const choice = await dialog.showMessageBox(win!, {
-      type: 'warning',
+    const msgOpts = {
+      type: 'warning' as const,
       buttons: ['Enable custom commands', 'Import without enabling', 'Cancel'],
       defaultId: 1,
       cancelId: 2,
@@ -99,7 +101,10 @@ export async function importBackup(win: BrowserWindow | undefined): Promise<Back
         'Custom commands run extra yt-dlp flags on every download. Only enable them if you trust this backup file.\n\n' +
         preview +
         more
-    })
+    }
+    const choice = await (win
+      ? dialog.showMessageBox(win, msgOpts)
+      : dialog.showMessageBox(msgOpts))
     if (choice.response === 2) return { ok: false } // Cancel — change nothing
     applyCustomCommands = choice.response === 0
   }

src/main/download.ts

@@ -239,9 +239,7 @@ export function startDownload(wc: WebContents, opts: StartDownloadOptions): Star
   if (missingBins.length > 0) {
     return {
       ok: false,
-      error:
-        `${missingBins.join(' and ')} not found in ${getBinDir()}\n` +
-        `Add the ffmpeg build's binaries to resources/bin/ (see the README there).`
+      error: `${missingBins.join(' and ')} not found in ${getBinDir()}. Add the ffmpeg build's binaries to resources/bin/ (see the README there).`
     }
   }
   // Reject anything that isn't an http(s) URL before it reaches yt-dlp's argv,

src/main/index.ts

@@ -22,6 +22,7 @@ import {
   type SystemThemeInfo,
   type TaskbarProgress
 } from '@shared/ipc'
+import { PAGE_BACKGROUND } from '@shared/theme'
 import { getYtdlpVersion, updateYtdlp, runStartupYtdlpAutoUpdate } from './ytdlp'
 import { getFfmpegVersions } from './ffmpeg'
 import { checkForAppUpdate, downloadAppUpdate, runAppUpdate } from './updater'
@@ -109,12 +110,6 @@ if (is.dev && devScript) {
 
 let mainWindow: BrowserWindow | null = null
 
-// Page background per theme — keep in sync with `pageBackground` in
-// src/renderer/src/theme.ts. Set as the window's NATIVE background so the
-// one-frame compositor repaint (when a tooltip/dropdown overlay first paints on
-// Windows) shows the app's current color instead of a mismatched white flash.
-const THEME_BACKGROUND = { light: '#f7f7f8', dark: '#161618' } as const
-
 // Resolve 'system' against the OS preference so callers always get a concrete color.
 function resolveBackgroundMode(theme: Settings['theme']): 'light' | 'dark' {
   return theme === 'system' ? (nativeTheme.shouldUseDarkColors ? 'dark' : 'light') : theme
@@ -177,7 +172,7 @@ function createWindow(): void {
     minWidth: 640,
     minHeight: 480,
     show: false,
-    backgroundColor: THEME_BACKGROUND[resolveBackgroundMode(getSettings().theme)],
+    backgroundColor: PAGE_BACKGROUND[resolveBackgroundMode(getSettings().theme)],
     autoHideMenuBar: true,
     webPreferences: {
       preload: join(__dirname, '../preload/index.cjs'),
@@ -342,7 +337,7 @@ function registerIpcHandlers(): void {
     // caption always matches the in-app theme (W3). Use the validated result.
     if (partial.theme) {
       BrowserWindow.fromWebContents(e.sender)?.setBackgroundColor(
-        THEME_BACKGROUND[resolveBackgroundMode(result.theme)]
+        PAGE_BACKGROUND[resolveBackgroundMode(result.theme)]
       )
       applyNativeTheme(result.theme)
     }
@@ -394,7 +389,7 @@ function registerIpcHandlers(): void {
     // background and title bar in sync the same way settingsSet does (W3).
     if (result.ok) {
       const theme = getSettings().theme
-      win?.setBackgroundColor(THEME_BACKGROUND[resolveBackgroundMode(theme)])
+      win?.setBackgroundColor(PAGE_BACKGROUND[resolveBackgroundMode(theme)])
       applyNativeTheme(theme)
     }
     return result
@@ -461,7 +456,7 @@ function registerIpcHandlers(): void {
 function registerSystemThemeBridge(): void {
   nativeTheme.on('updated', () => {
     const info = getSystemThemeInfo()
-    const bg = THEME_BACKGROUND[resolveBackgroundMode(getSettings().theme)]
+    const bg = PAGE_BACKGROUND[resolveBackgroundMode(getSettings().theme)]
     for (const win of BrowserWindow.getAllWindows()) {
       win.webContents.send(IpcChannels.systemThemeUpdate, info)
       if (getSettings().theme === 'system') win.setBackgroundColor(bg)

src/renderer/src/theme.ts

@@ -1,3 +1,4 @@
+import { PAGE_BACKGROUND } from '@shared/theme'
 import {
   createLightTheme,
   createDarkTheme,
@@ -168,14 +169,7 @@ export function getTheme(accent: AccentColor, mode: 'light' | 'dark'): Theme {
   return mode === 'dark' ? preset.dark : preset.light
 }
 
-// Page background behind the app shell. Light: a clean, barely-cool off-white.
-// Dark: neutral charcoal. Intentionally accent-INDEPENDENT — the accent shows up
-// only on buttons/links, so this stays neutral whichever preset is chosen.
-// Keep light/dark in sync with THEME_BACKGROUND in src/main/index.ts.
-export const pageBackground = {
-  light: '#f7f7f8',
-  dark: '#161618'
-}
+export const pageBackground = PAGE_BACKGROUND
 
 // Per-kind thumbnail tints. Video vs audio differ by ramp DEPTH rather than a
 // competing hue, keeping a monochrome look. Theme-aware (light/dark) but

src/shared/theme.ts

@@ -0,0 +1,11 @@
+/**
+ * Native window background colors per theme mode. Light: barely-cool off-white.
+ * Dark: neutral charcoal. These match the renderer's page background so there's
+ * no flash when a compositor overlay (tooltip, dropdown) first renders on Windows.
+ * Intentionally accent-independent — the accent shows on buttons/links only.
+ *
+ * Shared between main/index.ts (BrowserWindow backgroundColor) and
+ * renderer/src/theme.ts (CSS backgroundColor on the root div) so the two never
+ * drift apart.
+ */
+export const PAGE_BACKGROUND = { light: '#f7f7f8', dark: '#161618' } as const