feat: 'Show CLI sessions' toggle in Settings (#61)

Adds a server-side boolean setting (default: false) that controls whether
CLI sessions from state.db appear in the sidebar. Off by default so the
sidebar is clean until the user explicitly opts in.

- api/config.py: add show_cli_sessions to _SETTINGS_DEFAULTS and _SETTINGS_BOOL_KEYS
- api/routes.py: gate get_cli_sessions() call on the setting at request time
- static/index.html: checkbox in settings panel with description
- static/panels.js: load/save checkbox, refresh session list on save
- static/boot.js: load on startup alongside send_key and show_token_usage

Co-authored-by: Nathan Esquenazi <nesquena@gmail.com>

static/boot.js

@@ -308,7 +308,7 @@ document.querySelectorAll('.suggestion').forEach(btn=>{
 
 (async()=>{
   // Load send key preference
-  try{const s=await api('/api/settings');window._sendKey=s.send_key||'enter';window._showTokenUsage=!!s.show_token_usage;}catch(e){window._sendKey='enter';window._showTokenUsage=false;}
+  try{const s=await api('/api/settings');window._sendKey=s.send_key||'enter';window._showTokenUsage=!!s.show_token_usage;window._showCliSessions=!!s.show_cli_sessions;}catch(e){window._sendKey='enter';window._showTokenUsage=false;window._showCliSessions=false;}
   // Fetch active profile
   try{const p=await api('/api/profile/active');S.activeProfile=p.name||'default';}catch(e){S.activeProfile='default';}
   // Update profile chip label immediately