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>

src/main/badge.ts

@@ -0,0 +1,87 @@
+import { deflateSync } from 'zlib'
+import { nativeImage, type NativeImage } from 'electron'
+
+// CRC32 table polynomial used by PNG.
+function crc32(buf: Buffer): number {
+  let crc = 0xffffffff
+  for (const b of buf) {
+    crc ^= b
+    for (let i = 0; i < 8; i++) crc = crc & 1 ? (crc >>> 1) ^ 0xedb88320 : crc >>> 1
+  }
+  return (crc ^ 0xffffffff) >>> 0
+}
+
+function pngChunk(type: string, data: Buffer): Buffer {
+  const t = Buffer.from(type, 'ascii')
+  const len = Buffer.alloc(4)
+  len.writeUInt32BE(data.length)
+  const crc = Buffer.alloc(4)
+  crc.writeUInt32BE(crc32(Buffer.concat([t, data])))
+  return Buffer.concat([len, t, data, crc])
+}
+
+/**
+ * Build a 16×16 RGBA PNG containing a solid-colour filled circle. Used for the
+ * Windows taskbar overlay badge. No external deps — pure Node.js (Buffer + zlib).
+ */
+function makeDotPng(r: number, g: number, b: number): NativeImage {
+  const W = 16,
+    H = 16
+  const px = Buffer.alloc(W * H * 4, 0) // transparent bg
+  const cx = W / 2,
+    cy = H / 2,
+    rad = W / 2 - 1.5
+
+  for (let y = 0; y < H; y++) {
+    for (let x = 0; x < W; x++) {
+      const dx = x + 0.5 - cx,
+        dy = y + 0.5 - cy
+      if (dx * dx + dy * dy <= rad * rad) {
+        const i = (y * W + x) * 4
+        px[i] = r
+        px[i + 1] = g
+        px[i + 2] = b
+        px[i + 3] = 255
+      }
+    }
+  }
+
+  // PNG: IHDR (8-bit RGBA, no interlace), filter-0 rows, deflated IDAT, IEND.
+  const ihdr = Buffer.alloc(13)
+  ihdr.writeUInt32BE(W, 0)
+  ihdr.writeUInt32BE(H, 4)
+  ihdr[8] = 8 // bit depth
+  ihdr[9] = 6 // colour type: RGBA
+
+  const rows = Buffer.alloc(H * (1 + W * 4))
+  for (let y = 0; y < H; y++) {
+    rows[y * (1 + W * 4)] = 0 // filter type: None
+    px.copy(rows, y * (1 + W * 4) + 1, y * W * 4, (y + 1) * W * 4)
+  }
+
+  const sig = Buffer.from([137, 80, 78, 71, 13, 10, 26, 10])
+  const png = Buffer.concat([
+    sig,
+    pngChunk('IHDR', ihdr),
+    pngChunk('IDAT', deflateSync(rows)),
+    pngChunk('IEND', Buffer.alloc(0))
+  ])
+  return nativeImage.createFromBuffer(png)
+}
+
+// Lazy-initialised singletons — created on first use so this module is safe to
+// import before the app is ready (nativeImage.createFromBuffer is fine post-ready).
+let activeBadge: NativeImage | null = null
+let errorBadge: NativeImage | null = null
+
+/** Green dot badge for the taskbar overlay — shown when downloads are active. */
+export function getActiveBadge(): NativeImage {
+  if (!activeBadge) activeBadge = makeDotPng(58, 185, 108) // #3ab96c
+  return activeBadge
+}
+
+/** Red dot badge for the taskbar overlay — shown when a download has errored. */
+export function getErrorBadge(): NativeImage {
+  if (!errorBadge) errorBadge = makeDotPng(217, 48, 37) // #d93025
+  return errorBadge
+}