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/index.html

@@ -20,6 +20,12 @@
             <!-- a11y.29: a heading level was being used for type size. -->
             <p class="subtitle">Music how it was meant to bee.</p>
         </hgroup>
+        <!-- Global back/forward (#6). Before the library filter so the
+             two navigation controls in this bar are adjacent, and after
+             the brand because that is where a window's chrome ends and
+             the app's begins. Hidden below 600px by index.css: the
+             phone has a system back, and this bar has no room. -->
+        <nav-history></nav-history>
         <library-filter></library-filter>
         <search-bar></search-bar>
         <job-indicator></job-indicator>