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/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