fix(welcome): gate Claude and Opencode buttons on CLI availability too

Extends the Gemini gating from bb7fb9e to the other welcome-screen
buttons that had the same problem: shown unconditionally even when the
underlying CLI isn't installed.

- Add isClaudeAvailable() (claude-cli-resolver.ts) and GET
  /api/claude/status, mirroring the existing opencode/codex/gemini
  resolvers and status endpoints.
- Opencode already had a working /api/opencode/status the welcome
  screen just wasn't checking; wire it up the same way.
- Refactor loadGeminiAvailability() into a shared
  _loadCliAvailability(buttonId, statusUrl) helper instead of
  duplicating the fetch/try-catch three times.

Run-mode dropdown entries (Opencode/Codex) are intentionally left
unconditional here — follow-up PR.

src/web/public/terminal-ui.js

@@ -1195,6 +1195,8 @@ Object.assign(CodemanApp.prototype, {
     if (overlay) {
       overlay.classList.add('visible');
       this.loadTunnelStatus();
+      this.loadClaudeAvailability();
+      this.loadOpencodeAvailability();
       this.loadGeminiAvailability();
       this.loadHistorySessions();
       this.initSearchPanel();