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/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)