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/settings.js

@@ -29,7 +29,7 @@
         gameplay: {
             server: ['master_difficulty', 'av_offset_ms', 'miss_penalty',
                 'fail_behavior', 'countdown_before_song', 'default_arrangement'],
-            local: ['lefty', 'autoplayExit', 'arrangementNamingMode', 'countdownBeforeSong'],
+            local: ['lefty', 'autoplayExit', 'showUpNext', 'arrangementNamingMode', 'countdownBeforeSong'],
             after: function () {
                 // Left-handed is held on the highway object, not re-derived
                 // from localStorage on load — flip it back to the default.