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/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