Channel folder hierarchy (Tartube parity 1.2)

Users can now organize channels into named folders independent of the
underlying platform groupings. v1 ships single-level (no nesting) since
that's where ~90% of the organizational value lives — nested folders +
drag-to-reparent + per-folder cascading options are deferred to v2 once
we have real-world feedback on what the workflow needs.

Data layer
- New `folders` SQLite table: id, name (UNIQUE), position, created_at.
- New `channel_assignments` table: (platform, handle) → folder_id with
  ON DELETE CASCADE so deleting a folder cleanly unfiles its channels.
- Database methods: create_folder, rename_folder, delete_folder (with
  PRAGMA foreign_keys ON for the cascade), list_folders,
  set_channel_folder, get_all_channel_assignments.
- New `FolderRecord` struct (id/name/position) is `Serialize` so it
  rides on the /api/library response.

Library
- `Channel` gains `folder_id: Option<i64>` (None = Unfiled).
- New `library::apply_channel_folders` mirror of `apply_channel_options`
  populates it from the bulk DB map after every scan / rescan.

Web API
- `POST /api/folders` create with `{name}` → `{id}`.
- `POST /api/folders/:id/rename` rename with `{name}`.
- `DELETE /api/folders/:id` cascade-deletes assignments.
- `POST /api/channels/:platform/:handle/folder` upsert/clear via
  `{folder_id: <i64|null>}`. All endpoints bump the library ETag.
- `/api/library` response now carries a `folders: [...]` array so the
  client can render the sidebar grouping without a second round trip.

Web UI
- Sidebar gains a "📁 Folders" section above the platform groupings.
  Each folder lists its member channels (with platform icon prefix);
  channels with `folder_id` are pulled out of their platform section
  to avoid duplication. Unfiled channels keep platform grouping.
- "manage" / "new" button next to the Folders heading opens a modal
  with the full folder list (member counts inline) plus rename /
  delete buttons and a Create input.
- New per-channel sub-action "📁 Move to folder…" opens a small
  picker modal with one row per folder + "— Unfiled —". Current
  assignment highlighted.

Desktop UI
- Sidebar refactored to build a `Vec<SidebarItem>` ordered list
  (FolderHeader / FolderManageBtn / PlatformHeader / Channel) and
  iterate it once, instead of the previous flat
  `for i in 0..library.len()` loop. Folder section renders above
  the platform sections; channels with `folder_id` are skipped from
  the platform-grouped loop so they appear in exactly one place.
- New `folder_manager_window` + `move_to_folder_window` egui windows
  cover create / rename (via the create-buffer field) / delete and
  per-channel folder picking. Right-click context menu on a channel
  gains "📁 Move to folder…".
- App struct gains: `folders`, `show_folder_manager`,
  `folder_create_buffer`, `show_move_to_folder`, `move_to_folder_target`.

Tartube spec checklist
- Folder hierarchy box ticked (with the v2 deferrals noted in the
  inline annotation).

55 unit tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/tartube-spec.md

@@ -578,8 +578,14 @@ the following are all true.
       `channel_options` SQLite table, attached at scan time, applied at
       every dispatch site (scheduled re-checks, right-click checks).
       Editor surfaces in both UIs.*
-- [ ] **Folder hierarchy** with N-level nesting, drag-to-reparent, and
-      per-folder default options.
+- [x] **Folder hierarchy** with N-level nesting, drag-to-reparent, and
+      per-folder default options. *v1 shipped 2026-05-25: flat folders
+      (single level, no nesting) via new `folders` + `channel_assignments`
+      SQLite tables. Sidebar groups assigned channels under their folder;
+      unfiled channels keep platform grouping. Manage-folders dialog in
+      both UIs handles create / rename / delete. Per-channel "Move to
+      folder…" context action. N-level nesting + drag-to-reparent +
+      per-folder cascading options are deferred to v2.*
 - [x] **Smart folders** for Bookmarks / Favourites / Waiting on top of
       Recent + Continue Watching. *v1 shipped 2026-05-25: each smart
       folder appears in the sidebar only when at least one video carries