feat(audit): Batch 10 — CC conventions (shared line-buffer, cleanError)

The contained consistency consolidations; the sweeping ones are deferred with
their standard documented (they're the audit's own post-1.0 refactors, and
several would be reckless to do unattended).

Landed:
- src/main/lib/lineBuffer.ts (createLineBuffer): one newline line-splitter for
  streamed child-process output, replacing the duplicated buffer loop in
  download.ts and terminal.ts (CC3/CC4/SIMP5). Pure + unit-tested
  (test/lineBuffer.test.ts, 7 cases: partial lines, CRLF, empty lines, flush,
  multi-line chunks). download.ts keeps byte-identical marker parsing and no
  close-flush (yt-dlp template lines are always newline-terminated).
- ytdlp.ts: getYtdlpVersion/updateYtdlp now run stderr through cleanError, the
  same `cleanError(r.stderr) || r.error?.message || ''` idiom already used by
  download/probe/indexer (CC6) — a failed check shows the trailing error line.
- CL6 resolved: the one real redundancy was L15; clearDir/openFile/showInFolder
  are deliberate view-model wrappers (CC13), left as-is.

Deferred with documented standard + rationale (CODE-AUDIT.md): CC1 (breaking
persisted-key rename → migration), CC5+CL4 (security-critical updater
net.request, not live-verifiable here), CC7 (cosmetic arg-order), CC9 (zod dep),
CC10 (electron-store↔jsonStore split is deliberate for DPAPI), CC11 (config.ts
move), CC12 (file-tree reorg), CC13→L93, CC14→L142.

Verified: typecheck (node+web) + 275 tests + eslint + production build green;
touched files prettier-clean.

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

src/main/ytdlp.ts

@@ -2,6 +2,7 @@ import { existsSync, mkdirSync, copyFileSync } from 'fs'
 import { dirname } from 'path'
 import { getYtdlpPath, getBundledYtdlpPath, YTDLP_MISSING_MSG } from './binaries'
 import { execFileAsync } from './lib/exec'
+import { cleanError } from './log'
 import { VERSION_TIMEOUT_MS, YTDLP_UPDATE_TIMEOUT_MS } from './constants'
 import { getSettings, setSettings } from './settings'
 import { shouldAutoCheckYtdlp } from './ytdlpPolicy'
@@ -43,9 +44,11 @@ export async function getYtdlpVersion(): Promise<YtdlpVersionResult> {
 
   const r = await execFileAsync(ytdlpPath, ['--version'], { timeout: VERSION_TIMEOUT_MS })
   if (!r.ok) {
+    // Run stderr through the shared cleaner so yt-dlp's noisy output collapses to
+    // its trailing error line, consistent with download/probe/indexer (CC6).
     const msg = r.timedOut
       ? 'Timed out running yt-dlp.'
-      : (r.stderr || r.error?.message || '').trim()
+      : cleanError(r.stderr) || r.error?.message || ''
     return { ok: false, error: msg }
   }
   return { ok: true, version: r.stdout.trim() }
@@ -81,7 +84,7 @@ export async function updateYtdlp(channel: YtdlpUpdateChannel): Promise<YtdlpUpd
   if (!r.ok) {
     const msg = r.timedOut
       ? 'Timed out updating yt-dlp.'
-      : (r.stderr || r.error?.message || '').trim()
+      : cleanError(r.stderr) || r.error?.message || ''
     return { ok: false, error: msg }
   }
   return { ok: true, output: r.stdout.trim() }