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

backend/shortcuts/config.go

@@ -24,10 +24,19 @@ func DefaultBindings() map[string]string {
 		"player.repeat":      "R",
 		"player.mute":        "M",
 
-		// Navigation (Global scope)
+		// Navigation (Global scope).  Back and forward are the browser's
+		// own combination on every platform, which is the whole design
+		// brief for them: the app has one global history and this is the
+		// gesture people already have for it.  The modifier is what keeps
+		// them clear of `player.seekBack`/`seekForward`, which are the
+		// bare arrows -- a binding is matched on its full canonical
+		// string, so "Alt+Left" and "Left" are different keys and not a
+		// conflict.
 		"nav.search":    "/",
 		"nav.searchAlt": "Ctrl+F",
 		"nav.queue":     "Q",
+		"nav.back":      "Alt+Left",
+		"nav.forward":   "Alt+Right",
 
 		// App actions
 		"app.selectAll": "Ctrl+A",