feat(terminal): add WebGL renderer toggle in settings

Adds a 'WebGL Renderer' toggle to Settings > Appearance (desktop). WebGL
stays on by default; users can turn it off to force the DOM renderer when
they hit GPU glitches, without needing the ?nowebgl URL param. Explicit
opt-in (or ?webgl=force) clears a stale auto-fallback marker. Mobile skip
and the long-task auto-fallback safety net are unchanged.

The device/param/sticky/pref interaction is factored into a pure,
unit-tested shouldSkipWebGL() helper in constants.js.

src/web/public/index.html

@@ -939,6 +939,13 @@
                   <option value="og">OG Codeman</option>
                 </select>
               </div>
+              <div class="settings-item" id="appSettingsWebglRendererItem" title="Use the GPU-accelerated WebGL terminal renderer (desktop only). Turn off to force the DOM renderer if you hit GPU glitches. Codeman also auto-falls-back to the DOM renderer after repeated GPU stalls.">
+                <span class="settings-item-label">WebGL Renderer</span>
+                <label class="switch switch-sm">
+                  <input type="checkbox" id="appSettingsWebglRenderer">
+                  <span class="slider"></span>
+                </label>
+              </div>
               <!-- Input Section -->
               <div class="settings-section-header">Input</div>
               <div class="settings-item settings-item-multiline" title="Shows typed characters instantly via overlay while forwarding keystrokes to the server in the background. Enables Tab completion, preserves input across tab switches, and protects against session crashes. Recommended for mobile and high-latency connections.">