feat(shell): take the top bar out of the phone's layout

The row is deleted from the grid template below 600px, not the header
hidden. That is 3.25em of a 439 CSS px viewport -- the single biggest
vertical win the reference device has to give, and the reason the issue
asks for the row rather than for a smaller bar.

Each of the five things the bar held has somewhere else to be there:
nav-history is the platform's own back gesture and was already gone from
899 down, the job indicator is <job-band> (#62, which is what this was
blocked on), the search box is a modal opened from the view's own
header, the library filter is Settings -> Libraries, and the wordmark
stays where it is.

Three things are load-bearing.

**The header is visually hidden rather than display: none**, because
that h1 is the document's top-level heading and several pages have no
other one -- page-header renders no h1 when its heading is empty, and
Settings has no page-header at all. Its four controls are display: none
*inside* it, which is what keeps them out of the tab order: a
visually-hidden container is still focusable, and tabbing into a search
box nobody can see is worse than not having one.

**The fit pass stands down**, from the bar's computed position rather
than from a width. With the bar out of flow there is no content box to
measure children against, and a pass that ran would collapse the
wordmark on every resize and report success about a 1px box.

**top-bar-fit.spec.ts keeps 390 and asserts the stronger property.**
"Nothing hangs out of the bar" is trivially true of a bar with no row
and would pass on a build that merely broke it, so what that width asks
now is that the content starts where the row above it ends. Measuring
against the window instead would have been asserting "and no background
job is running", which that spec is not about and cannot arrange.

Closes #57

frontend/src/services/top-bar-fit.ts

@@ -119,6 +119,16 @@ export const FIT_STEPS: readonly FitStep[] = [
  * @returns the ids collapsed, in the order they were given up.
  */
 export function measureTopBarFit(bar: HTMLElement): string[] {
+    // Below 600px there is no bar to fit (#57): `index.css` takes it
+    // out of the grid and leaves it visually hidden at 1px, carrying
+    // nothing but the document's `h1`. Measuring that reports the
+    // wordmark as overflowing 1px of content box and collapses it every
+    // time -- true, and about nothing, since the whole bar is already
+    // invisible. Asking the *computed position* rather than the
+    // viewport width is what keeps this file free of a breakpoint the
+    // stylesheet already owns.
+    if (getComputedStyle(bar).position === 'absolute') return [];
+
     const fits = () => {
         const style = getComputedStyle(bar);
         const box = bar.getBoundingClientRect();