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/Views/Prototypes/LibraryView.swift

@@ -147,7 +147,17 @@ struct LibraryView: View {
         LibraryTagStrip(
             filters: $filters,
             selection: $selectedFilter,
-            available: unusedTags
+            // The prototype has no tag picker sheet behind it; + pins the
+            // heaviest tag that isn't already open, which is enough to exercise
+            // the strip's layout.
+            onBrowseTags: {
+                guard let tag = unusedTags.first else { return }
+                let new = LibraryFilter(name: tag, tag: tag)
+                withAnimation(.spring(duration: 0.35, bounce: 0.1)) {
+                    filters.append(new)
+                    selectedFilter = new.id
+                }
+            }
         )
     }