feat: AI-driven browser tests — Tests tab + browser_test agent tool

New feature: give Studio's AI control of any browser tab to test buttons,
read the console, and report which buttons work / fail.

Backend (internal/api/browser_test.go, ~480 LOC):
- WebSocket endpoint /api/ws/browser-test, auth by single-use 5-min token
- BrowserTestStore: session map (capped at 16, LRU evict), token store
- REST: /api/test/snippet (issues token + JS snippet), /api/test/sessions,
  /api/test/console/{id}
- Agent tool 'browser_test' wired into the registry, with actions:
  list_clickables / click / eval / console / current_url / type / wait /
  summary. click returns the console_delta produced during the click.
- Embedded JS runner: opens WS, hooks console + window.onerror +
  unhandledrejection, dispatches dispatcher commands, replies with
  correlation IDs, watches for URL changes.

Frontend:
- New Tests tab (web/src/components/Tests.jsx): snippet copy + connected
  sessions list + live console viewer
- App.jsx: 5th tab + Ctrl+4 shortcut (Config moves to Ctrl+5)
- api/client.js: getTestSnippet / getTestSessions / getTestConsole

Studio prompt:
- internal/agent/prompts/studio_system.md: added browser_test entry to the
  tools table + <browser_test_strategy> section explaining the recommended
  loop (summary → list_clickables → click → check console_delta → report)

Versioning:
- v0.6.0 → v0.7.0
- CHANGELOG.md: full entry under v0.7.0

web/src/components/App.jsx

@@ -1,5 +1,5 @@
 import { useState, useEffect, useCallback, useMemo, useRef } from 'react'
-import { LayoutDashboard, Sparkles, Terminal, Settings } from 'lucide-react'
+import { LayoutDashboard, Sparkles, Terminal, Settings, TestTube2 } from 'lucide-react'
 import api from '../api/client'
 import { getTheme, applyTheme } from '../themes'
 import { useI18n } from '../i18n'
@@ -7,6 +7,7 @@ import Dashboard from './Dashboard'
 import Studio from './Studio'
 import Shell from './Shell'
 import Config from './Config'
+import Tests from './Tests'
 import OnboardingWizard from './OnboardingWizard'
 
 export default function App() {
@@ -24,6 +25,7 @@ export default function App() {
     { id: 'dash', label: t('tabs.dashboard'), icon: <LayoutDashboard size={15} /> },
     { id: 'studio', label: t('tabs.studio'), icon: <Sparkles size={15} /> },
     { id: 'shell', label: t('tabs.shell'), icon: <Terminal size={15} /> },
+    { id: 'tests', label: 'Tests', icon: <TestTube2 size={15} /> },
     { id: 'config', label: t('tabs.config'), icon: <Settings size={15} /> },
   ], [t])
 
@@ -54,7 +56,8 @@ export default function App() {
         Digit1: 'dash',
         Digit2: 'studio',
         Digit3: 'shell',
-        Digit4: 'config',
+        Digit4: 'tests',
+        Digit5: 'config',
       }
       if (map[e.code]) {
         e.preventDefault()
@@ -92,6 +95,7 @@ export default function App() {
       { keys: layout.keys.enter, desc: t('statusbar.runCommand') },
       { keys: `${layout.keys.up}/${layout.keys.down}`, desc: t('statusbar.commandHistory') },
     ],
+    tests: [],
     config: [],
   }), [layout, t])
 
@@ -129,6 +133,7 @@ export default function App() {
         <div className={activeTab === 'dash' ? '' : 'tab-hidden'}><Dashboard api={api} refreshRef={dashRefreshRef} /></div>
         <div className={activeTab === 'studio' ? '' : 'tab-hidden'}><Studio api={api} /></div>
         <div className={activeTab === 'shell' ? '' : 'tab-hidden'}><Shell api={api} isSudo={isSudo} /></div>
+        <div className={activeTab === 'tests' ? '' : 'tab-hidden'}><Tests api={api} /></div>
         <div className={activeTab === 'config' ? '' : 'tab-hidden'}><Config api={api} /></div>
       </main>