Harden audit findings: correctness, type-safety, Windows conventions & polish

Audit-pass over CODE-AUDIT.md (~48 items closed this pass; all verified —
typecheck + 234 tests + eslint + prettier green).

Correctness / bugs:
- B3: match the release checksum to the asset's filename line (no wrong-hash verify)
- B4: newline-safe metadata probe (one --print with a unit-separator delimiter)
- B5 / L88: guard the meta event against canceled items; progress no longer promotes
  a queued item outside pump()
- B7: cookie-login promise always resolves (handles destroy-without-close)
- L146: trim parser rejects >2 colon-group times; M36: Library selection counts only
  actionable rows
- L11 / L50 / L156 / L57 / L159 / L15 / L3: live queue count, empty-cookie message,
  schedule picker min, dead-code/comment cleanup

Type safety:
- Enable noUncheckedIndexedAccess + noFallthroughCasesInSwitch (15 real edge cases fixed)

Resilience / Windows / metadata:
- R5: settings write failure handled (no unhandled IPC rejection; reconciles to truth)
- W1 / W5 / W6: min window size, seeded folder picker, parented sign-in window;
  L147 dead macOS branches removed
- CL1: shared stdout markers; package/builder metadata (license, homepage, repository,
  copyright, tsbuildinfo glob)

Copy / docs / tests:
- M37 / SR9 dev-jargon cleanup in hints; M8 / M25 / M26 / L66 / L80 / L81 reconciled
- New unit tests for L35 (isValidMediaItem) and L36 (compareVersions)

This commit also checkpoints the previously-uncommitted feat/tray-background-clipboard
work it builds on: background running + auto-download, library clipboard detection,
tray, binary management & library scale, credential encryption at rest, the shared
jsonStore and ui/ primitives, and the eslint/prettier tooling.

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

eslint.config.js

@@ -0,0 +1,43 @@
+import js from '@eslint/js'
+import tseslint from 'typescript-eslint'
+import globals from 'globals'
+import prettier from 'eslint-config-prettier'
+
+// Flat config (ESLint 9 + typescript-eslint 8). Codifies the existing house style
+// (CC2): typescript-eslint's recommended logic/type rules, with formatting left
+// entirely to Prettier (eslint-config-prettier turns off any stylistic rules).
+// Run with `npm run lint`; `npm run format` applies Prettier.
+export default tseslint.config(
+  { ignores: ['out/**', 'dist/**', 'node_modules/**', '**/*.cjs'] },
+  js.configs.recommended,
+  ...tseslint.configs.recommended,
+  {
+    languageOptions: {
+      globals: { ...globals.node, ...globals.browser }
+    },
+    rules: {
+      // Empty catch blocks are a deliberate best-effort pattern across the app.
+      'no-empty': ['error', { allowEmptyCatch: true }],
+      // Allow a documented @ts-ignore. The preload's window fallback needs one
+      // whose necessity is incremental-build-state-dependent, so @ts-expect-error
+      // (which errors when momentarily unused) is the wrong tool there.
+      '@typescript-eslint/ban-ts-comment': [
+        'error',
+        { 'ts-ignore': 'allow-with-description', minimumDescriptionLength: 3 }
+      ],
+      // Allow intentionally-unused names prefixed with `_` (e.g. IPC event args).
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        { argsIgnorePattern: '^_', varsIgnorePattern: '^_' }
+      ]
+    }
+  },
+  {
+    // TypeScript already resolves identifiers/types, so core no-undef only
+    // false-positives on globals and type-only references here.
+    files: ['**/*.{ts,tsx}'],
+    rules: { 'no-undef': 'off' }
+  },
+  // Keep Prettier last so it wins over any formatting-related rule.
+  prettier
+)