fix(shell): give the top bar a measured fit at every width

The bar was 611px inside a 600px viewport at the bottom of the Compact
band, and 862px while a scan with a real library's title ran, because
`job-indicator` is `hidden` when idle and 235px wide when it is not.
`body` is `overflow-x: auto`, so a user got a horizontal scrollbar on a
shell #24 promised would not need one — and the band is 600 to 899 with
work in flight, not the 600 to 610 the idle measurement suggested.

`services/top-bar-fit.ts` is `page-header`'s treatment one bar up: a
ResizeObserver, every pass starting from all-visible, hiding the
lowest-priority child until it fits. Measured rather than breakpointed
because three of the five children are as wide as their content — the
library filter by the longest library name, the indicator by the
running job's title, the search box by its view-scoped placeholder — so
any width picked is right for one library, one job and one view.

What yields is decided by #24's own sentence, which rules out the two
cheapest candidates in the Direction. Hiding the library filter takes
away an action, since it is the only control in the app that selects a
library (filed as #148, which is the phone already doing it), and
collapsing search to an icon is #57's, which is blocked behind #62. So
the wordmark yields first — a brand the window title bar repeats, and
visually-hidden rather than `display: none` because that h1 is the
document's heading — and then the indicator's label, leaving the ring,
which the component already does below 600px and whose live region
announces the state either way.

"Fits" is the children against the content box, not `scrollWidth`
against `clientWidth`: `scrollWidth` counts the left padding and not
the right, so the first version read 700/700 with the indicator sitting
in the whole right gutter. And the bar does not resize when a job
starts, which is the case this is for, so every child is observed too.

Pinned before it was fixed, as the issue asks. On the unfixed build the
new spec fails at 600 idle and at 600, 800 and 900 with a job, and
passes at 390, 899 and 1440; `layout-overflow.spec.ts` gains 600x600
and failed there. That spec asserts on the *shell*, so it was green
throughout this defect — the per-child measurement is #69's lesson, and
it is what caught the gutter case above.

Closes #143

frontend/index.css

@@ -104,6 +104,42 @@ p {
     flex: 0 1 320px;
 }
 
+/* What the bar gives up when it does not fit is decided by measuring
+   it (`services/top-bar-fit.ts`, #143). Two rules here are what make
+   that measurement mean anything.
+
+   **Nothing but the search box may shrink.** `scrollWidth` reports a
+   perfect fit while a child quietly truncates -- #69's trap, one
+   component over -- and the indicator's label is `text-overflow:
+   ellipsis`, so it would have absorbed the deficit and hidden it. The
+   search box is exempt because it shrinks between its 320px basis and
+   the 200px floor its own stylesheet sets, and a narrower input hides
+   nothing it was showing. */
+.top-bar hgroup,
+.top-bar library-filter,
+.top-bar job-indicator {
+    flex-shrink: 0;
+}
+
+/* **The wordmark yields its width, not its existence.** It is the
+   app's top-level heading as well as its brand, and `display: none`
+   would take a document from one `h1` to none at exactly the widths
+   where the view's own header is the only thing left saying where you
+   are. This is `styles/sr-only.css.ts`'s recipe, written out because
+   that one is a `CSSResult` for shadow roots and this is the light
+   DOM. */
+.top-bar hgroup.yj-collapsed {
+    position: absolute;
+    width: 1px;
+    height: 1px;
+    padding: 0;
+    margin: -1px;
+    overflow: hidden;
+    clip-path: inset(50%);
+    white-space: nowrap;
+    border: 0;
+}
+
 /* 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