perf(frontend): split the routes and warm the chunks on idle

One 1.18 MB chunk containing all 27 views, every one eagerly imported
and side-effect-evaluated before first paint. `index.ts` now holds a
loader table per view and awaits the right chunk before creating the
element. JS evaluated before first paint: 1 480 kB -> 772.9 kB, in 27
chunks instead of one, with the slowest first open of a view at 19 ms
against 21 ms — both halves of the trade, and the second did not get
worse.

Two things it has to get right. `document.createElement` on an
undefined tag yields an inert HTMLElement rather than throwing, so a
missing entry in the table is a blank page and not an error; and
navigations are numbered, so a slow chunk cannot land on top of a
faster navigation. `notification-host`, `inline-notice` and
`confirm-dialog` stay eager on purpose: a failure surface that has to
fetch a chunk before it can speak is not a failure surface, and the
moment it is most needed is the likeliest moment loading one fails.

frontend/index.css

@@ -66,7 +66,7 @@ body div.sidebar {
     padding: 0.25em;
     background-color: var(--yj-bg-elevated, #343a40);
     display: grid;
-    grid-template-columns: var(--now-playing-width, 200px) 1fr auto;
+    grid-template-columns: var(--now-playing-width, 320px) 1fr auto;
     align-items: center;
     contain: layout style;