docs(admin): dashboard page — stats, watcher, Quick Actions, scan panel

Second page of the admin tree. Documents the four-card stats grid with
what each number actually counts (instance-wide totals; Books = every
media item including podcast episodes), the File Watcher card's data
source (/api/scanner/watch/status, armed by Auto-Scan) and its
zero-libraries meaning, Force Rescan All Libraries with its real
mechanics (one job per library, skip-on-active, backfill tool vs.
routine shallow refresh), and the Scanning panel's live progress +
Cancel all scans. Links only to pages that exist (scanning.md); the
Settings reference stays prose until that page lands.

The real-tree nav test's Administration assertion loosens further: guide
leads, everything else strictly follows usabilityOrder — durable across
the remaining page landings. Also fixes the amend this commit replaces:
its first version briefly carried a forward link to the unwritten
settings.md (caught by the link gate, now removed) — the gate worked;
the && chain around go test let the bad commit through, which is exactly
why this amend exists.

docs/admin/dashboard.md

@@ -0,0 +1,72 @@
+<!-- Ground truth: templates/admin.templ (stats grid, File Watcher card,
+     Quick Actions, Scanning panel), internal/router/frontend.go (/admin
+     handler), internal/router/admin_library.go (getAdminStats),
+     web/src/admin.ts (scanAllLibraries one-job-per-library, watch status
+     fetch, progress rows), internal/handlers/scanner.go (watch status,
+     coalescing 409). Re-verify against these when the page changes. -->
+
+# Admin Dashboard
+
+The dashboard at `/admin` is the admin landing page: a four-number health
+check, the file-watcher status, and the place where library-wide scans are
+launched and watched.
+
+## The stats grid
+
+Four counts, straight from the database, refreshed on every page load:
+
+| Card | What the number counts |
+| --- | --- |
+| **Libraries** | Every library — book, comic, audiobook, and podcast libraries alike |
+| **Books** | Every item across all of them: books, comics, audiobooks, and podcast episodes |
+| **Users** | Every account on the instance |
+| **Devices** | Every registered sync device (Android, KOReader, Kobo) |
+
+These are instance-wide totals — they don't filter by what any individual
+user can see.
+
+## File Watcher
+
+The **File Watcher** card shows how many libraries are currently under
+live watch for file changes. The watcher exists when **Auto-Scan** is
+enabled on the Settings page (Settings → Scanning); the number updates on
+page load.
+
+- Watching **0** libraries usually means auto-scan is off — turn it on in
+  Settings → Scanning, and the watcher arms on the next server start.
+- Watched libraries get shallow scans as files change, plus a periodic
+  poll on the scan interval — see [Scanning](scanning.md) for exactly what
+  those do.
+
+## Quick Actions
+
+### Force Rescan All Libraries
+
+The big one. This is a **force rescan of every library**: each file is
+re-read and re-extracted with the scanner's current parsing, and hash
+identifiers are recomputed — nothing unchanged is skipped. Use it as the
+backfill tool after mass edits or scanner improvements; don't use it as a
+routine refresh (that's what the watcher and [per-library
+scans](scanning.md) are for, which skip unchanged files).
+
+Mechanically it starts **one scan job per library** and reports per
+library: started, or skipped because that library already had a scan
+running (only one scan per library can run at a time).
+
+### Manage Libraries
+
+A shortcut to [Libraries](libraries.md), where folders, access, and
+per-library scans live.
+
+## The Scanning panel
+
+Whenever scans are running, a **Scanning Libraries** panel appears with a
+live progress row per library (progress bar, current file counts, and a
+per-library cancel button), plus:
+
+- **Cancel all scans** — stops every queued or running scan job
+- The close button — hides the panel; scans keep running without it
+
+The panel appears for any scan — the dashboard's own force rescan, or one
+started from the Libraries page. Progress updates live while the page is
+open; the jobs themselves run on the server either way.