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/buildArgs.ts

@@ -106,8 +106,14 @@ export const ARIA2C_ARGS = 'aria2c:-x 16 -s 16 -k 1M'
 /**
  * Shell-like split of a raw "extra yt-dlp args" string (Phase C custom-command
  * templates) into argv tokens. Supports single- and double-quoted spans so a
- * flag value containing spaces (e.g. a --ppa recipe) can be typed as one
- * token; no escape-character or nested-quote support beyond that.
+ * flag value containing spaces (e.g. a --ppa recipe) can be typed as one token.
+ *
+ * LIMITATIONS (audit M2): there is no escape-sequence support (`\"`, `\'`) and
+ * quotes cannot nest — a literal quote inside a value can't be expressed, and an
+ * unterminated quote falls through to the `\S+` branch and captures the quote
+ * char itself. For the yt-dlp use cases this targets (proxy URLs, filename
+ * patterns, ffmpeg recipes), that's sufficient. If a future template needs
+ * nested or escaped quotes, redesign this to a proper shlex or JSON-based format.
  */
 export function parseExtraArgs(raw: string): string[] {
   const args: string[] = []