feat(webui): add limit bars, forecasts, events, exports and URL state

The Usage tab shows a bar for each configured limit. It turns amber at
80% and red at 100%. The tab also shows rejected requests per day, a
projected month-end cost with budget headroom, and CSV export for users
and groups. Names link to Charts with the matching scope.

The Charts tab ranks models over the full 24h, 30d or 12mo window. A
provider filter is applied on the server, and the ranking can be
exported as CSV. A new Events tab lists recent failures, filtered by
kind and user. The selected tab, filters and scope live in the URL
hash. Polling stops while the page is hidden. Views show which replica
answered, and a banner lists configuration warnings.

CSV cells that start with a formula character are neutralised. Links
sit outside the accordion buttons, so both controls keep their
accessible names. admin_assets_gen.go is regenerated.

Claude-Session: https://claude.ai/code/session_01V5jYJNxpDFdJkyyExpADrv

webui/src/assets/main.css

@@ -54,6 +54,7 @@
     --color-chart-tokens-out: var(--chart-tokens-out);
     --color-chart-cost: var(--chart-cost);
     --color-status-warn: var(--status-warn);
+    --color-bar-amber: var(--bar-amber);
     --radius-sm: calc(var(--radius) - 4px);
     --radius-md: calc(var(--radius) - 2px);
     --radius-lg: var(--radius);
@@ -109,6 +110,27 @@
      * light --card measures ~5.06:1.
      */
     --status-warn: #a35c00;
+
+    /*
+     * --bar-amber: F1 (usage-vs-limit bars, UsageBar.vue) — the filled
+     * amber segment of a budget bar at BAR_AMBER_THRESHOLD (lib/usage-
+     * bars.ts). The track it fills against is --muted (UsageBar.vue's
+     * `bg-muted` on the bar's own background span) — WCAG 1.4.11 non-text
+     * contrast requires 3:1 against that ADJACENT color.
+     *
+     * P8 review fix: this used to match --chart-tokens-out's hue/value
+     * (#e69f00) on the reasoning that it was "already proven legible at
+     * small UI sizes on both --card and --muted" — that reasoning was
+     * never actually checked against --muted specifically, and #e69f00
+     * measures (WCAG relative-luminance formula, oklch(0.95 0.006 75)
+     * --muted as the track) 1.95:1 there, well under the 3:1 this comment
+     * itself claimed. Reusing --status-warn's own light-mode value
+     * (#a35c00) instead measures 4.44:1 against --muted — comfortably
+     * clears 3:1, and keeps every semantic amber in this panel visually
+     * consistent (same value already used for the "degraded" badge tier,
+     * ProviderRateBadge.vue) rather than adding a third amber shade.
+     */
+    --bar-amber: #a35c00;
 }
 
 @media (prefers-color-scheme: dark) {
@@ -147,6 +169,9 @@
            reader auditing this block sees every semantic-color token's
            dark value in one place. */
         --status-warn: #f0ac1f;
+
+        /* Same reasoning as --chart-tokens-out's own dark-mode comment above — legible as-is, redefined here anyway so every semantic-color token's dark value lives in one place. */
+        --bar-amber: #f0ac1f;
     }
 }