fix(v3): add "Show 'Up Next'" toggle so the player pill can be turned off (#612)

The v0.3.0 player chrome's persistent upcoming-section pill (#v3-upnext,
drawn by static/v3/player-chrome.js's updateUpNext) shipped with no off
switch: it always showed during playback whenever a section was upcoming,
overlapping the top-right FPS HUD and ignoring the 3D-highway "Show 'Up
Next' section card" checkbox (a different, in-canvas widget demoted to
default-off precisely because this pill is the canonical readout). Users
reading the pill as that same setting saw "disabled in settings but still
there."

Add a real core toggle, following the autoplayExit idiom:
- static/app.js: client-only `showUpNext` localStorage pref (absence =
  enabled), _showUpNextEnabled()/setShowUpNext(), loadSettings()
  hydration, and a read-only window.feedBack.showUpNext getter. Disabling
  mid-playback hides the pill immediately.
- static/v3/index.html: a "Show 'Up Next'" switch in the Gameplay tab.
- static/v3/player-chrome.js: gate updateUpNext() on the pref.
- static/v3/settings.js: add showUpNext to RESET_MAP.gameplay.local.

Default ON, so behaviour is unchanged for existing users. v3-only (the
pill is v3 core chrome); no Tailwind rebuild.


Claude-Session: https://claude.ai/code/session_01QbexxfTt8q2tAn436MqGWF

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

static/v3/index.html

@@ -505,6 +505,20 @@
                             </label>
                         </div>
                     </div>
+                    <!-- "Up Next" pill -->
+                    <div class="fb-srow">
+                        <span class="fb-srow-icon"><svg fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M13 5l7 7-7 7M5 5l7 7-7 7"/></svg></span>
+                        <div class="fb-srow-main">
+                            <div class="fb-srow-title">Show &ldquo;Up Next&rdquo;</div>
+                            <div class="fb-srow-desc">Display the upcoming-section pill in the top-right of the player during playback.</div>
+                        </div>
+                        <div class="fb-srow-control">
+                            <label class="fb-switch">
+                                <input type="checkbox" id="setting-show-upnext" checked onchange="setShowUpNext(this.checked)">
+                                <span class="fb-switch-track"></span>
+                            </label>
+                        </div>
+                    </div>
                 </div>
             </div>