feat(web-ui): better stats and ergonomics

Usage page: add 7-day trend chart (activity/tokens/cost tabs),
framework breakdown panel with per-framework run/tool/error counts
and proportional bars, and 7d aggregate pills above the chart.

Dashboard: add avg cost/run metric pill to the metrics strip.

Run detail: extract and display prompt preview from the first agent
span's payload above the spans table.

Bug fixes: stat-list bars now render correctly (flex-direction:column),
right-panel-tab active background uses correct accent color, missing
framework colors added for hermes/codex/gemini/copilot. Dead code
renderSessionRow removed from sessions.js. Hardcoded font-family
replaced with CSS variable in metric-pill-value and token-stat-value.
Usage page cleanup() wired into router teardown.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/web-ui/static/modules/router.js

@@ -13,7 +13,7 @@ import { renderRun, cleanup as cleanupRunDetail }             from './pages/run-
 import { renderAgents, cleanup as cleanupAgents }             from './pages/agents.js';
 import { renderInfrastructure, cleanup as cleanupInfra }      from './pages/infrastructure.js';
 import { renderSettings }                                     from './pages/settings.js';
-import { renderUsage }                                        from './pages/usage.js';
+import { renderUsage, cleanup as cleanupUsage }              from './pages/usage.js';
 
 // Exported so all page modules can write into it without querying the DOM each time
 export const app = document.getElementById('app');
@@ -31,6 +31,7 @@ export function cleanupLiveViews() {
   cleanupSessionDetail();
   cleanupRunDetail();
   cleanupDashboard();
+  cleanupUsage();
 }
 
 export function route() {