fix(a11y): name account controls per account, stop hardcoding white on accent

A-04: every account rendered a folder toggle labelled "Toggle account folders",
so three accounts gave three controls a screen reader reads identically. The
address is in the label now, and the account link is named explicitly rather
than by a `truncate` span that CSS can cut mid-address.

A-07 (second half): computing --rgb-accent-ink by luminance achieves nothing
while 63 call sites paint `text-white` on `bg-accent` directly. They now use
text-accent-ink and inherit the measured choice. The two left behind are white
over a black gradient and over a danger fill, which are not the accent.

H-05: the search box kept a stale query when a folder was clicked — the input
survives the Turbo visit and a folder page has no `q` to overwrite it with, so
the box claimed a filter the list below it had not applied. It now tracks `?q=`
in the address bar, which is also right on the way back in.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

templates/compose/_no_account.html.twig

@@ -14,7 +14,7 @@ to give it. A dead end with a way out beats a stack trace.
     <p class="mt-1.5 text-sm text-ink-muted">{{ 'compose.no_account.body'|trans }}</p>
 
     <a href="{{ path('app_settings_index', { section: 'accounts' }) }}"
-       class="mt-5 inline-flex items-center justify-center h-9 px-4 rounded-lg text-sm font-medium text-white
+       class="mt-5 inline-flex items-center justify-center h-9 px-4 rounded-lg text-sm font-medium text-accent-ink
               bg-accent hover:bg-accent-strong transition-colors">
         {{ 'compose.no_account.cta'|trans }}
     </a>