fix(webui): accessible warn token, day-window model badges, review minors

Review round (v0.22) — MUST-1, SHOULD-2's frontend half, and every
folded minor that touched webui/.

MUST-1 (contrast): the degraded badge's amber previously reused
--color-chart-tokens-out, tuned for a chart mark, not body text —
measured ~2.22:1 against --card in light mode, a WCAG 1.4.3 failure on
exactly the "something's wrong" tier. A new semantic --status-warn token
pair (main.css, with a @theme inline mapping) replaces it: #a35c00 light
(~5.06:1), #f0ac1f dark (~8.97:1, already passing, redefined anyway so
every semantic-color token's dark value lives in one place).
ProviderRateBadge.vue's degraded tier now uses border-status-warn/40
text-status-warn.

SHOULD-2 (frontend half — see the backend commit for admin.go/limits.go):
AdminModelRateView drops attemptsMinute/failuresMinute (the backend no
longer sends them at all). ProviderRateBadge.vue's attempts-minute/
failures-minute props become optional; their absence (not a passed-
through 0) triggers a new dayRateTitle fallback (lib/provider-rate.ts) —
a real "0 attempts this minute" and "no minute data fetched at all" read
identically as 0 otherwise, which would misreport a model badge as having
live-minute data it never measured.

Folded minors:
- formatRatePercent floors instead of rounds — only a true 100% rate
  (zero failures) ever renders "100%"; 99.6% now shows "99%", not a
  round-tripped "100%" that would overstate it.
- providerSuccessRate NaN-guards both inputs (Number.isFinite).
- ProvidersView's modelRateFor uses `p.modelRates?.[model]` — a stale
  client against an older server build could otherwise throw rather than
  fall back, matching its own doc comment's stated intent.
- ProviderRateBadge's title is now also its aria-label — title alone is
  mouse-only; a keyboard/screen-reader user previously got none of the
  minute/day detail.
- The accordion's "Last refresh" DETAIL row (not just the trigger's
  compact text) now shares refreshLabel's three-state logic via a new
  refreshDetailLabel (lib/format.ts) — it previously called
  formatTimestamp directly, so a discovery-off provider's expanded row
  still said "never" beside a trigger that correctly said "discovery
  off".
- providerModelScopeID's Go doc comment (limits.go, backend commit)
  cross-references configNamePattern for why the "/" join is collision-
  safe even though a model id may itself contain "/".
- useTabHash.ts documents its two intentional URL-normalization quirks:
  case-sensitive hash matching, and an invalid/absent initial hash
  normalizing only in memory, not in the address bar, until the next tab
  change.

webui/src/assets/main.css

@@ -53,6 +53,7 @@
     --color-chart-tokens-in: var(--chart-tokens-in);
     --color-chart-tokens-out: var(--chart-tokens-out);
     --color-chart-cost: var(--chart-cost);
+    --color-status-warn: var(--status-warn);
     --radius-sm: calc(var(--radius) - 4px);
     --radius-md: calc(var(--radius) - 2px);
     --radius-lg: var(--radius);
@@ -86,6 +87,20 @@
     --chart-tokens-in: #0072b2;
     --chart-tokens-out: #e69f00;
     --chart-cost: #d55e00;
+
+    /*
+     * --status-warn: a semantic "degraded, not broken" indicator (the
+     * Providers tab's amber success-rate badge, ProviderRateBadge.vue) —
+     * deliberately its OWN token, not a reuse of --chart-tokens-out despite
+     * sharing that token's hue family. --chart-tokens-out is tuned to read
+     * against a chart's plot background at a small mark size, not as body
+     * text on --card; measured here at #e69f00-on-card it comes out to
+     * ~2.22:1, well under WCAG 1.4.3's 4.5:1 text-contrast minimum — a
+     * genuine accessibility bug on exactly the badge tier meant to say
+     * "something's wrong" (MUST-1, v0.22 review round). #a35c00 on the
+     * light --card measures ~5.06:1.
+     */
+    --status-warn: #a35c00;
 }
 
 @media (prefers-color-scheme: dark) {
@@ -116,6 +131,14 @@
         --chart-tokens-in: #4098d7;
         --chart-tokens-out: #f0ac1f;
         --chart-cost: #e8763a;
+
+        /* #f0ac1f on the dark --card measures ~8.97:1 — no separate dark
+           value was needed to clear WCAG 1.4.3 here, unlike light mode
+           (see --status-warn's own doc comment above), but the token is
+           still redefined per-mode rather than left to fall through, so a
+           reader auditing this block sees every semantic-color token's
+           dark value in one place. */
+        --status-warn: #f0ac1f;
     }
 }