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.css

@@ -104,6 +104,15 @@ p {
     flex: 0 1 320px;
 }
 
+/* The bar is `justify-content: space-between`, which with four children
+   spreads them evenly and left back/forward floating in the middle of
+   nothing. Collecting the free space *after* this one puts the pair
+   beside the brand, where a browser keeps them, and leaves the
+   right-hand group exactly as it was. */
+.top-bar nav-history {
+    margin-right: auto;
+}
+
 ul {
     list-style-type: none;
 }
@@ -133,6 +142,23 @@ ul {
     .subtitle {
         display: none;
     }
+
+    /* Back/forward is Desktop-band chrome (#6), and 900 is the same
+       line the sidebar's labels and the subtitle are already given up
+       at -- below it the shell is narrow enough that the header is
+       what runs out of room first. Measured at 600, the bottom of the
+       Compact band: the bar is 611px inside a 600px viewport *before*
+       this component exists (filed separately), and 695px with it, so
+       keeping it here would be widening a violation of the promise
+       that nothing scrolls sideways at a supported size.
+
+       Nothing is unreachable as a result, which is the rule that
+       decides it: Alt+Left / Alt+Right are global and every width has
+       them, the detail views keep their own back buttons, and the
+       phone additionally has the platform's gesture. */
+    .top-bar nav-history {
+        display: none;
+    }
 }
 
 body div.sidebar {
@@ -346,7 +372,13 @@ body div.sidebar {
 
     /* The search box is the one header control worth its width; the
        library filter is a rarely-changed setting and reachable from
-       the drawer's Settings. */
+       the drawer's Settings.
+
+       `nav-history` is already gone from 899 down. It would belong
+       here anyway and for a stronger reason than width: the phone has
+       Back as a gesture or a button the OS owns, and this app hooks it
+       (`popstate`), so a second Back in the chrome duplicates a
+       control the platform provides. */
     .top-bar library-filter {
         display: none;
     }