Apply all CODE-AUDIT.md findings (S1-S5, P1-P3, M1-M3)

Security:
- S1 (backup.ts): require explicit confirmation before enabling custom-command
  templates from a backup file; import templates in a disabled state if declined
- S2 (cookies.ts): deny non-http/https popups in cookie login window, matching
  the main window's setWindowOpenHandler defence
- S3 (download.ts): main-process concurrency cap — startDownload now rejects
  spawns when active.size >= maxConcurrent (defence-in-depth; renderer already
  enforces this but should not be the only gate)
- S4 (settings.ts, validation.ts): reject filenameTemplate values containing
  path traversal (.. segments or absolute paths); validate outputDir is absolute
- S5 (history.ts, errorlog.ts, templates.ts, validation.ts): per-field
  validation on JSON reads — invalid entries dropped rather than blindly trusted

Performance:
- P1 (settings.ts): getSettings() now only writes to disk when sanitization
  actually changed a value; removes synchronous file I/O on every hot-path read
- P2 (ipc.ts, download.ts, downloads.ts): renderer forwards its pre-probed
  metadata (title/channel/duration) via StartDownloadOptions.meta; main uses
  it directly instead of spawning a redundant second yt-dlp probe

Maintainability:
- M1 (log.ts, download.ts, probe.ts): extract duplicated cleanError() to
  src/main/log.ts; both callers import from the shared module
- M2 (buildArgs.ts): document parseExtraArgs escape-sequence limitations
- M3 (electron-builder.yml): clarify icon TODO as a pre-release action
- P3 (downloads.ts): document that lowering maxConcurrent mid-flight does
  not pause active downloads (intended behaviour, now written down)

Tests:
- Add test/validation.test.ts covering isSafeFilenameTemplate, isSafeOutputDir,
  isValidHistoryEntry, isValidErrorLogEntry, isTemplateLike (76 tests pass)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main/errorlog.ts

@@ -2,6 +2,7 @@ import { app } from 'electron'
 import { join } from 'path'
 import { readFileSync, writeFileSync, existsSync } from 'fs'
 import type { ErrorLogEntry } from '@shared/ipc'
+import { isValidErrorLogEntry } from './validation'
 
 // Plain JSON in userData, same shape as history.ts. Persisted so a failure
 // report survives the queue item being cleared (Seal's "debug report").
@@ -11,11 +12,14 @@ function errorLogFile(): string {
   return join(app.getPath('userData'), 'errorlog.json')
 }
 
+// Per-entry validation (isValidErrorLogEntry, in validation.ts) so a hand-edited
+// or corrupted errorlog.json can't feed the UI entries with the wrong shape —
+// invalid rows are dropped. (audit S5)
 export function listErrorLog(): ErrorLogEntry[] {
   try {
     if (!existsSync(errorLogFile())) return []
     const data = JSON.parse(readFileSync(errorLogFile(), 'utf8'))
-    return Array.isArray(data) ? (data as ErrorLogEntry[]) : []
+    return Array.isArray(data) ? data.filter(isValidErrorLogEntry) : []
   } catch {
     return []
   }