feat(shortcuts): tell the key story once, and give the arrows back

Decision 1 keeps the unmodified single-key bindings, and Settings was
the only place they were written down — three of the four categories of
them, because config-page listed the categories by hand, so the autotag
keys were written down nowhere at all. `?` now opens an overlay from
anywhere the app owns the keyboard, and both surfaces read one table
(services/shortcut-meta.ts, moved out of config-page's private static).

The other half is the same explanation from the other side. Phase 1
gave the arrow keys to the grid, correctly — but all six of them, and
no list in this app moves horizontally: track-list's own handler and
utils/roving-rows both take Up/Down/Home/End and ignore Left/Right. So
seeking stopped working from a focused row and nothing gained the keys.
Reproduced in the running app: two ArrowRights on a focused track row,
zero Player.Seek calls, against one per press from the body.

A shifted character no longer reports Shift, so the binding is `?` and
not `Shift+?` — the character already carries the shift, and a layout
where it does not is a layout where "Shift+?" is wrong anyway.

backend/shortcuts/config.go

@@ -29,8 +29,9 @@ func DefaultBindings() map[string]string {
 		"nav.searchAlt": "Ctrl+F",
 		"nav.queue":     "Q",
 
-		// App actions (Global scope, Ctrl modifier)
+		// App actions
 		"app.selectAll": "Ctrl+A",
+		"app.shortcuts": "?",
 
 		// Panel-specific (track list).  There is no `tracklist.delete`:
 		// it was bound to Delete and advertised in Settings as