Replace the Tags tab with a tag picker sheet

The tab bar drops from five to four; Tags now opens as a sheet from the +
in the library's filter strip, which is the control that was already
about adding a tag.

Picking a tag pins it as a filter tab rather than pushing to a separate
per-tag list. That made TagBookmarksView redundant — the filter tab shows
the same thing, in whichever layout you're already using — so it's gone.

This also fixes what the + could reach. The old inline menu listed tags
found on the loaded page, so it offered 21 of 96 tags, and once a tag
filter was active it collapsed to just the tags co-occurring with that
one — pinning a second unrelated tag was impossible. The sheet sources
names from the tags endpoint via a new BookmarksViewModel.loadAllTags(),
and counts still come from loaded bookmarks, so a tag we haven't paged in
shows no number rather than a wrong one.

The sheet is searchable, marks already-pinned tags with a check, and
picking a pinned tag selects that tab instead of duplicating it.

Verified by driving the UI: the tab bar is now Bookmarks/Sources/
Podcasts/Search, + opens the sheet with 42 rows where the old menu had
21, and picking "ai" pinned an "ai" tab. Suite passes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JgLHztZGaEHvS3KNeGQmRM

Marks/MarksApp.swift

@@ -54,9 +54,6 @@ struct MainContainer: View {
             Tab("Bookmarks", systemImage: "bookmark", value: AppTab.bookmarks) {
                 BookmarksView(viewModel: viewModel, onDisconnect: onDisconnect)
             }
-            Tab("Tags", systemImage: "tag", value: AppTab.tags) {
-                TagsView(viewModel: viewModel)
-            }
             Tab("Sources", systemImage: "tray.full", value: AppTab.sources) {
                 SourcesView(
                     library: sourceLibrary,