fix(html): make remaining mouse-only controls keyboard accessible

[Ayaan Gazali (ayaangazali)]

Rationale

Four controls are still reachable only with a mouse, the same shape fixed for tabs in #41434, chips
in #42149, and expandable titles and the image diff switcher in #42310 and #42311.

location	before	consequence
headerView.tsx:136	<div role='button' title='Settings'>	theme/settings dialog unreachable
testFilesView.tsx:84	<div className='metadata-toggle' role='button'>	metadata cannot be expanded
labels.tsx:31	bare <span className='label' onClick>	tag filter chips unreachable
actionList.tsx:115	bare <div className='action-list-show-all' onClick>	cannot clear the time-range filter

The first two are the worse pair: they declare role="button" and then cannot take focus, so
assistive tech is told there is a button that cannot be operated. The other two carry no role at
all, so they are not announced as controls in the first place.

All four become native buttons with a :focus-visible ring, no key handlers. The html reporter
files use --color-accent-fg, matching chip.css and tabbedPane.css; actionList.css uses
--vscode-focusBorder, matching the rest of the trace viewer. The gear also gains
aria-haspopup="dialog" and aria-expanded, and the Metadata toggle gains aria-expanded, which
neither had.

labels.tsx only becomes a button on the clickable branch. Label has an href branch that
already renders a real anchor, but LabelsClickView calls it without href, so the tag chips
always took the bare-span path.

One thing I deliberately did not change.actionList.tsx:167, the error/warning badge that
reveals the console, is also mouse-only. I made it a button, and it broke
ui-mode-trace.spec.ts:778: because the badge sits inside a treeitem, giving it an accessible
name appends that name to every action row, - treeitem "Evaluate 2ms Reveal console". That is a
regression in the accessibility tree rather than an improvement, so it needs a different approach
and is better done separately than bundled here.

Test

headerView.spec.ts gets one keyboard case for the gear: focus it, assert focus lands, press Enter,
assert aria-expanded flips. On current main it fails at toBeFocused with Received: inactive.

Green: html-reporter component tests 18, reporter-html.spec.ts 198, trace-viewer.spec.ts 113,
ui-mode-trace.spec.ts 27, flint clean.

Fixes#42323

---

for context, my last three PRs here were closed with the CLA check still pending, so if that is
what blocks this one too please just say so and I will get it sorted rather than have it linger.
also, reading how you structured titleSuffix in #42310 and the --vscode-focusBorder fallback in
#42311 is what made me check the token per package here instead of copying one blindly, so thanks
for that. freshman in college, learning a lot from these reviews :)

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