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.

frontend/src/services/shortcut-meta.ts

@@ -0,0 +1,166 @@
+/**
+ * What each shortcut action is called, where it applies, and what it is
+ * bound to out of the box.
+ *
+ * This lived as a private static in `config-page`, which meant Settings
+ * was the only place in the app the key bindings were written down —
+ * and Settings renders three of the four categories, so the autotag
+ * keys were written down nowhere at all. The `?` overlay and the
+ * Settings editor now read one table.
+ */
+
+export interface ShortcutMeta {
+    label: string;
+    category: string;
+    scope: string;
+    defaultKey: string;
+}
+
+/** The categories, in the order both surfaces show them. */
+export const SHORTCUT_CATEGORIES = [
+    'Player',
+    'Navigation',
+    'App',
+    'Autotag',
+] as const;
+
+export const SHORTCUT_META: Record<string, ShortcutMeta> = {
+        'player.playPause': {
+            label: 'Play / Pause',
+            category: 'Player',
+            scope: 'global',
+            defaultKey: 'Space',
+        },
+        'player.next': {
+            label: 'Next Track',
+            category: 'Player',
+            scope: 'global',
+            defaultKey: 'N',
+        },
+        'player.previous': {
+            label: 'Previous Track',
+            category: 'Player',
+            scope: 'global',
+            defaultKey: 'P',
+        },
+        'player.volumeUp': {
+            label: 'Volume Up',
+            category: 'Player',
+            scope: 'global',
+            defaultKey: 'Up',
+        },
+        'player.volumeDown': {
+            label: 'Volume Down',
+            category: 'Player',
+            scope: 'global',
+            defaultKey: 'Down',
+        },
+        'player.seekForward': {
+            label: 'Seek Forward',
+            category: 'Player',
+            scope: 'global',
+            defaultKey: 'Right',
+        },
+        'player.seekBack': {
+            label: 'Seek Back',
+            category: 'Player',
+            scope: 'global',
+            defaultKey: 'Left',
+        },
+        'player.shuffle': {
+            label: 'Toggle Shuffle',
+            category: 'Player',
+            scope: 'global',
+            defaultKey: 'S',
+        },
+        'player.repeat': {
+            label: 'Cycle Repeat',
+            category: 'Player',
+            scope: 'global',
+            defaultKey: 'R',
+        },
+        'player.mute': {
+            label: 'Toggle Mute',
+            category: 'Player',
+            scope: 'global',
+            defaultKey: 'M',
+        },
+        'nav.search': {
+            label: 'Focus Search',
+            category: 'Navigation',
+            scope: 'global',
+            defaultKey: '/',
+        },
+        'nav.searchAlt': {
+            label: 'Focus Search (Alt)',
+            category: 'Navigation',
+            scope: 'global',
+            defaultKey: 'Ctrl+F',
+        },
+        'nav.queue': {
+            label: 'Toggle Queue',
+            category: 'Navigation',
+            scope: 'global',
+            defaultKey: 'Q',
+        },
+        'app.shortcuts': {
+            label: 'Keyboard Shortcuts',
+            category: 'App',
+            scope: 'global',
+            defaultKey: '?',
+        },
+        'app.selectAll': {
+            label: 'Select All',
+            category: 'App',
+            scope: 'global',
+            defaultKey: 'Ctrl+A',
+        },
+        'tracklist.play': {
+            label: 'Play Selected',
+            category: 'Navigation',
+            scope: 'panel:track-list',
+            defaultKey: 'Enter',
+        },
+        'autotag.apply': {
+            label: 'Apply Match',
+            category: 'Autotag',
+            scope: 'panel:autotag',
+            defaultKey: 'A',
+        },
+        'autotag.skip': {
+            label: 'Skip Folder',
+            category: 'Autotag',
+            scope: 'panel:autotag',
+            defaultKey: 'S',
+        },
+        'autotag.leave': {
+            label: 'Leave As Is',
+            category: 'Autotag',
+            scope: 'panel:autotag',
+            defaultKey: 'L',
+        },
+        'autotag.paste': {
+            label: 'Paste Release URL',
+            category: 'Autotag',
+            scope: 'panel:autotag',
+            defaultKey: 'U',
+        },
+        'autotag.search': {
+            label: 'Search Candidates',
+            category: 'Autotag',
+            scope: 'panel:autotag',
+            defaultKey: 'F',
+        },
+        'autotag.next': {
+            label: 'Next Folder',
+            category: 'Autotag',
+            scope: 'panel:autotag',
+            defaultKey: 'Down',
+        },
+        'autotag.previous': {
+            label: 'Previous Folder',
+            category: 'Autotag',
+            scope: 'panel:autotag',
+            defaultKey: 'Up',
+        },
+    };