feat(shell): global back and forward in the top bar

The history stack has been global since the Android back gesture landed
-- every navigation is an entry and `popstate` restores any of them in
either direction. What the report describes as "back is tab-scoped" is
that the only way back was a detail view's own button, which leaves the
screen with the view it belongs to: click over to Tracks and the album
you were reading is still one entry away with nothing on screen saying
so.

`<nav-history>` is that affordance, plus `nav.back` / `nav.forward` on
Alt+Left / Alt+Right -- the browser's own combination, and clear of the
bare arrows that seek, since a binding matches on its full canonical
string.

Forward is not back negated, which is why the old `pushedEntries`
counter is gone rather than extended: `popstate` carries no direction
and fires identically both ways, so one counter decremented on every
pop reads a forward as a second back. Each entry carries its index and
the shell keeps the current one and a high-water mark, which also
survives a jump of more than one.

The buttons dispatch the events the rest of the app already dispatches
rather than calling `history` themselves -- the shell owns the guard
that stops a press at the root leaving the app, and a second caller
reaching for history is how the old `navStack` came to disagree with
the platform.

Below 900px the control stands down: the top bar is what runs out of
room first below that, and nothing becomes unreachable -- the shortcuts
are global at every width and the phone has the platform's gesture.

Closes #6

frontend/src/services/keyboard-shortcut-service.ts

@@ -400,6 +400,20 @@ async function dispatch(action: string): Promise<void> {
             break;
         }
 
+        // The keyboard half of #6. It dispatches the same events the
+        // header's buttons and the detail views' own back buttons do,
+        // rather than calling `history.back()` here: the shell owns the
+        // guard that stops a press at the root leaving the app, and a
+        // second caller reaching for `history` directly is how the old
+        // `navStack` came to disagree with the platform.
+        case 'nav.back':
+            document.dispatchEvent(new CustomEvent('navigate-back'));
+            break;
+
+        case 'nav.forward':
+            document.dispatchEvent(new CustomEvent('navigate-forward'));
+            break;
+
         case 'nav.queue': {
             const queuePanel = document.getElementById(
                 'queue-panel',

frontend/src/services/shortcut-meta.ts

@@ -103,6 +103,18 @@ export const SHORTCUT_META: Record<string, ShortcutMeta> = {
             scope: 'global',
             defaultKey: 'Q',
         },
+        'nav.back': {
+            label: 'Back',
+            category: 'Navigation',
+            scope: 'global',
+            defaultKey: 'Alt+Left',
+        },
+        'nav.forward': {
+            label: 'Forward',
+            category: 'Navigation',
+            scope: 'global',
+            defaultKey: 'Alt+Right',
+        },
         'app.shortcuts': {
             label: 'Keyboard Shortcuts',
             category: 'App',