Add Pinchflat-style media manager: index channels into playlist folders

Index an entire YouTube channel or playlist once, then download it into
organized <Channel>/<Playlist>/<NNN> - <Title> folders, with incremental
re-sync. Built in six rebuild-gated phases (F-K); see ROADMAP-PINCHFLAT.md.

- F: channel-walk indexer (/playlists + /videos) -> persisted Source +
  MediaItem JSON stores; pure classify/dedup logic in indexerCore.ts
- G: Windows-safe folder paths + dir sanitizer (collectionOutputTemplate)
- H: Library tab + source tree + "Download pending" into the existing queue
- I: state-preserving re-index merge + persist-downloaded-on-complete
- J: watched sources, RSS fast-check, Task Scheduler + --sync launch
  (the OS-level scheduling/RSS need a real-install smoke test)
- K: .info.json / thumbnail / .description sidecars for media servers

Also gitignore .gitea-token. 106 unit tests pass; typecheck + build clean.

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

src/main/validation.ts

@@ -7,7 +7,7 @@
  */
 
 import { isAbsolute } from 'path'
-import type { HistoryEntry, ErrorLogEntry, CommandTemplate } from '@shared/ipc'
+import type { HistoryEntry, ErrorLogEntry, CommandTemplate, Source, MediaItem } from '@shared/ipc'
 
 // --- Path-traversal sanitization (audit S4) ---------------------------------
 
@@ -75,3 +75,38 @@ export function isTemplateLike(o: unknown): o is CommandTemplate {
   const t = o as Record<string, unknown>
   return typeof t.id === 'string' || typeof t.id === 'number'
 }
+
+/** A persisted sources.json row must have the right shape or it's dropped on read. */
+export function isValidSource(o: unknown): o is Source {
+  if (!o || typeof o !== 'object') return false
+  const s = o as Record<string, unknown>
+  return (
+    typeof s.id === 'string' &&
+    typeof s.url === 'string' &&
+    (s.kind === 'channel' || s.kind === 'playlist') &&
+    typeof s.title === 'string' &&
+    typeof s.addedAt === 'number' &&
+    typeof s.itemCount === 'number' &&
+    isOptionalString(s.channel) &&
+    (s.lastIndexedAt === undefined || typeof s.lastIndexedAt === 'number')
+  )
+}
+
+/** A persisted media-items.json row must have the right shape or it's dropped on read. */
+export function isValidMediaItem(o: unknown): o is MediaItem {
+  if (!o || typeof o !== 'object') return false
+  const m = o as Record<string, unknown>
+  return (
+    typeof m.id === 'string' &&
+    typeof m.sourceId === 'string' &&
+    typeof m.videoId === 'string' &&
+    typeof m.title === 'string' &&
+    typeof m.url === 'string' &&
+    typeof m.playlistTitle === 'string' &&
+    typeof m.playlistIndex === 'number' &&
+    typeof m.downloaded === 'boolean' &&
+    isOptionalString(m.durationLabel) &&
+    isOptionalString(m.filePath) &&
+    (m.downloadedAt === undefined || typeof m.downloadedAt === 'number')
+  )
+}