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/test/stores/view-stores.test.ts

@@ -8,6 +8,7 @@ import { describe, expect, it, beforeEach } from 'vitest';
 
 import { searchStore } from '@store/search-store';
 import { activeViewStore } from '@store/active-view-store';
+import { historyStore } from '@store/history-store';
 import { trackListStore } from '@store/tracklist-store';
 import { exploreCache, ARTIST_IMAGE_CACHE_LIMIT } from '@store/explore-cache';
 import { Events } from '../../src/events';
@@ -133,6 +134,46 @@ describe('active view store', () => {
   });
 });
 
+describe('history store', () => {
+  beforeEach(() => {
+    historyStore.setDepth(false, false);
+  });
+
+  it('holds both answers, because forward is not back negated', () => {
+    historyStore.setDepth(true, false);
+
+    expect(historyStore.get()).toEqual({ canBack: true, canForward: false });
+
+    // The middle of the list: both directions available at once, which
+    // a single depth counter cannot express and which is the state the
+    // old `pushedEntries` got wrong.
+    historyStore.setDepth(true, true);
+
+    expect(historyStore.get()).toEqual({ canBack: true, canForward: true });
+  });
+
+  it('does not notify when neither answer changed', () => {
+    let notifications = 0;
+    const off = historyStore.subscribe(() => {
+      notifications += 1;
+    });
+
+    historyStore.setDepth(true, true);
+    historyStore.setDepth(true, true);
+    off();
+
+    expect(notifications).toBe(1);
+  });
+
+  it('starts with both unavailable, which is the truth at launch', () => {
+    // A fresh session is one entry deep and that entry is *replaced*,
+    // not pushed, so there is nothing of ours behind it. A control
+    // that assumed otherwise would offer a press that does nothing --
+    // and on Android, one the OS would have used to exit the app.
+    expect(historyStore.get()).toEqual({ canBack: false, canForward: false });
+  });
+});
+
 describe('track list store', () => {
   it('starts from the default column set', () => {
     expect(trackListStore.getState().columnIds.length).toBeGreaterThan(0);