feat(v3): add desktop-only "Start in fullscreen" system option

Adds a "Start in fullscreen" toggle to the Settings → System panel,
addressing the desktop request in feedBack-desktop#97: users want the
app to launch fullscreen without hitting the OS hotkey every time.

The block ships hidden and is gated exactly like the App-updates block:
setupWindowOptions() only unhides + wires it when the feedBack-desktop
bridge exposes window.feedBackDesktop.window.{getStartFullscreen,
setStartFullscreen}. Web/Docker builds have no such bridge, so the
section never appears there. Persistence lives desktop-side because
only the Electron main process can read the pref at window-creation
time — core just proxies through the bridge.

The desktop bridge + launch behaviour land in a follow-up
feedBack-desktop PR.

Signed-off-by: gionnibgud <gionnibgud@gmail.com>

static/v3/index.html

@@ -753,6 +753,21 @@
                             <a href="https://github.com/got-feedback/feedback-desktop/releases" target="_blank" rel="noopener" class="text-accent hover:text-accent-light underline">download new versions from GitHub Releases</a>.
                         </p>
                     </div>
+                    <!-- Window options — desktop-only; setupWindowOptions() unhides. -->
+                    <div id="window-options-block" class="hidden">
+                        <div class="fb-srow">
+                            <div class="fb-srow-main">
+                                <div class="fb-srow-title">Start in fullscreen</div>
+                                <div class="fb-srow-desc">Launch the desktop app in fullscreen every time.</div>
+                            </div>
+                            <div class="fb-srow-control">
+                                <label class="fb-switch">
+                                    <input type="checkbox" id="setting-start-fullscreen">
+                                    <span class="fb-switch-track"></span>
+                                </label>
+                            </div>
+                        </div>
+                    </div>
                     <!-- Library folder path -->
                     <div class="fb-srow fb-srow-stack">
                         <div class="fb-srow-main">