fix(ui): raise the last controls below the touch floor

The rest of #186's second table, and one thing it could not have said.

    .section-toggle          187x15  autotag
    .folders-menu-trigger     32x18  autotag
    .back-button              32x32  artist-details
    Requests / Downloads tabs 85x34, 96x34
    .search-mode-tab          89x26, 79x26  explore
    explore search input     325x18  in a 36px box

**back-button was six controls, not one.** The issue names it in
artist-details because that is the view the sweep opened; the same
declaration is byte-identical in artist-details, genre-details,
playlist-details, smart-playlist-details, explore-artist-details and
explore-album-details, 32px in all six. So it is styles/back-button.
css.ts now, adopted by each, and a source sweep fails on a seventh
copy -- because the failure this invites is not a size changing, it is
somebody adding a detail view and writing `.back-button` out again,
which no device sweep would catch for the same reason this one did
not. That is icon-language.test.ts's shape, and the argument for it
here is the inverse of the column arrows': one declaration covering
36 controls is cheap to fix, and six declarations of one control are
six chances to miss five.

It is a real 44px box rather than padding with the width handed back:
a detail header runs no fit pass, and this button has a visible
background, so a hit area larger than the circle would be a control
bigger than it looks. The size is #55's, reached there for the same
reason -- "the way out is 44px on a phone".

**The explore search box was two faults.** The row was 36px *and* the
input inside it was 18, so eight pixels at each edge were not a target
at all: a tap near the top of the box landed on the container and did
nothing. The container is 44 and the input stretches to it.

**The Downloads tabs take padding rather than a min-size**, because
the mark for the selected tab is its bottom border -- a min-size
centres the label and leaves the underline 10px beneath it.

page-action-check-now (113x29) is in that table and is not here: it is
a PageAction, so #195 raised it with the rest of the header's actions
and touch-targets.test.ts already covers it.

Verified on the device, sweeping each view the way the issue was
filed: explore, downloads, autotag and artist-details now report
**one** control under the floor apiece, and it is the skip link, which
#186 already ruled out as keyboard-only. .search-mode-tab 89x44 and
79x44, the search input 325x44, the Downloads tabs 85x44 and 96x44,
.section-toggle 174x44, .folders-menu-trigger 44x44, .back-button
44x44.

All 12 new tests fail on main, the source sweep naming all six copies.
make ui-test 1041 pass; make e2e 236 pass on chromium, which is half
an answer -- CI has the other half.

Closes #186

frontend/src/components/downloads-view/downloads-view.ts

@@ -97,21 +97,11 @@ export class DownloadsView extends ViewLifecycleMixin(LitElement) {
                route to the panel it names, so it is the last control
                that should be hard to hit -- and the underline that
                marks the active one is drawn on the bottom border,
-               which a taller box moves further from the label. So the
+               which a taller box moves further from the label. That
-               height goes on *padding*, keeping the border against
+               is why the height goes on padding rather than on a
-               the label rather than 10px below a centred one.
+               min-size: the border follows the padding down and the
-
+               label stays where the underline is. */
-               The min-size is the floor and is not redundant: padding
-               alone made this 44px here and **43px in CI**, because
-               the total is 13 + 13 + 2 + whatever line box the font
-               gives 13px text, and ubuntu:24.04's is a pixel shorter
-               than this machine's. A height computed from a font's
-               line box is not a height you control -- the same
-               mistake #195 made about a layout property measured on
-               one engine, one layer down, and caught here by the test
-               rather than by a person. */
             .tab {
-                min-block-size: 44px;
                 padding: 13px 14px;
                 font-size: 13px;
                 font-weight: 600;