Move File Management link into the bottom tab bar

Replace the dashboard-only footer link with a "Files" item in the
persistent bottom tab bar (Home/Notes/Help/Settings) on every page, so
it's reachable the same way everywhere instead of only from the
dashboard.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -88,10 +88,11 @@ silently stay blank instead of erroring.
 - Cover art: uses a manually-placed `cover`/`folder`/`thumbnail`/`thumb`/
   `poster` image if a course has one, otherwise auto-generates one via
   `ffmpeg` from a frame of the course's first video
-- **File Management** ([/unsorted](templates/unsorted.html), linked from
-  Settings and from a footer link on the dashboard): a dedicated page for
-  everything that touches files on disk, since it needs more room than a
-  Settings card and warrants review before anything actually changes.
+- **File Management** ([/unsorted](templates/unsorted.html), its own item
+  ("Files") in the bottom tab bar alongside Home/Notes/Help/Settings): a
+  dedicated page for everything that touches files on disk, since it needs
+  more room than a Settings card and warrants review before anything
+  actually changes.
   - *Sort Unsorted*: drop new/incoming courses into an `Unsorted` folder at
   the library root, then scan proposes a destination for each one by keyword
   overlap against the *existing* category tree - read fresh from disk every