feat: add voice input mic button via Web Speech API

- index.html: add #btnMic (hidden by default, shown if browser supports
  SpeechRecognition) and #micStatus listening indicator inside .composer-box

- boot.js: IIFE-scoped mic handler wired to Web Speech API
  * recognition.continuous=false (auto-stops after ~2s silence)
  * recognition.interimResults=true (live transcript preview in textarea)
  * Toggles .recording class + shows #micStatus while active
  * Handles 'not-allowed', 'no-speech', 'network' errors via showToast()
  * btnSend.onclick stops active recognition before sending
  * Entire feature disabled/hidden gracefully when API unavailable

- style.css: .mic-btn, .mic-btn.recording (red pulse animation),
  .mic-status, .mic-dot, @keyframes mic-pulse

- tests/test_sprint20.py: 46 tests covering HTML structure, CSS rules,
  JS logic, error handling, and regression checks (376 total, all pass)

No API keys, no external libraries, no server changes. Browser-only.
Works in Chrome, Edge, Safari (partial). Firefox unsupported (hides button).

static/style.css

@@ -187,6 +187,11 @@
   .icon-btn{width:34px;height:34px;border-radius:8px;background:none;border:none;color:var(--muted);cursor:pointer;display:flex;align-items:center;justify-content:center;font-size:16px;transition:all .15s;}
   .icon-btn{opacity:.75;}
   .icon-btn:hover{background:rgba(255,255,255,.08);color:var(--text);opacity:1;}
+  .mic-btn{transition:color .15s,background .15s;}
+  .mic-btn.recording{color:#e94560;background:rgba(233,69,96,.12);animation:mic-pulse 1.2s ease-in-out infinite;}
+  @keyframes mic-pulse{0%,100%{box-shadow:0 0 0 0 rgba(233,69,96,.3);}50%{box-shadow:0 0 0 6px rgba(233,69,96,0);}}
+  .mic-status{font-size:11px;color:#e94560;padding:4px 12px;display:flex;align-items:center;gap:6px;}
+  .mic-dot{width:6px;height:6px;border-radius:50%;background:#e94560;animation:mic-pulse 1.2s ease-in-out infinite;flex-shrink:0;}
   .status-text{font-size:11px;color:var(--muted);padding-left:4px;}
   .send-btn{padding:7px 18px;border-radius:10px;font-size:13px;font-weight:600;background:linear-gradient(135deg,#5ba8f5,#7cb9ff);border:none;color:#0a1628;cursor:pointer;display:flex;align-items:center;gap:6px;transition:all .15s;flex-shrink:0;letter-spacing:.01em;}
   .send-btn:hover{background:linear-gradient(135deg,#7cb9ff,#a0d0ff);transform:translateY(-1px);}