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/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,