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/probe.ts

@@ -2,6 +2,7 @@ import { execFile } from 'child_process'
 import { existsSync } from 'fs'
 import { getYtdlpPath } from './binaries'
 import { fmtBytes } from './download'
+import { cleanError } from './log'
 import { assertHttpUrl } from './url'
 import {
   BEST_FORMAT_ID,
@@ -136,15 +137,6 @@ function buildPlaylist(data: RawInfo): PlaylistInfo {
   }
 }
 
-function cleanError(stderr: string): string {
-  const lines = stderr
-    .split('\n')
-    .map((l) => l.trim())
-    .filter(Boolean)
-  const errLine = [...lines].reverse().find((l) => /^error/i.test(l))
-  return (errLine ?? lines[lines.length - 1] ?? '').replace(/^error:\s*/i, '').trim()
-}
-
 /**
  * Probe a URL with `yt-dlp -J --flat-playlist`. Resolves to a single video (with
  * its format list) or, when the URL is a playlist, the flat list of its entries.