Enable thinking selector in MessageEditor

- Import Select component from mini-lit and Brain icon from lucide
- Add thinkingLevel property to track current thinking level (off/minimal/low/medium/high)
- Enable showThinkingSelector (was disabled before)
- Add onThinkingChange callback for thinking level changes
- Render Select component with thinking level options when model supports reasoning
- Position thinking selector on left side next to attachment button
- Add i18n translations for Off/Minimal/Low/Medium/High in English and German
- Rename showThinking → showThinkingSelector in AgentInterface and ChatPanel for consistency
- Remove showDebugToggle property from AgentInterface (unused)
- Clean up browser-javascript tool output message (remove CSP note)

packages/web-ui/src/ChatPanel.ts

@@ -168,7 +168,7 @@ export class ChatPanel extends LitElement {
 							.session=${this.session}
 							.enableAttachments=${true}
 							.enableModelSelector=${true}
-							.enableThinking=${true}
+							.showThinkingSelector=${true}
 							.showThemeToggle=${false}
 							.showDebugToggle=${false}
 							.onApiKeyRequired=${this.onApiKeyRequired}