feat(v3): tabbed, card-row settings page + per-plugin settings category (#584)

Replace the single long scrolling v3 settings screen with a horizontal tab
bar (Gameplay / Audio / Graphics / Keybinds / Progression / Mic / Plugins /
System) over card rows (icon + title + description, control on the right) with
a per-category Reset.

- static/v3/index.html: tab bar + card-row markup (ids keep hydrating through
  the unchanged app.js loadSettings()/persistSetting() path).
- static/v3/settings.js (new): tab switching + active-tab persistence
  (localStorage 'v3-settings-tab'), per-category reset, read-only Keybinds
  reference from window.getAllShortcuts().
- static/v3/v3.css: plain CSS, no Tailwind rebuild.
- Per-plugin settings tab: new optional settings.category in plugin.json →
  plugins/__init__.py surfaces settings_category; app.js mounts each plugin
  <details> into #plugin-settings-<category> (fallback: Plugins tab).
  highway_3d ships category: "graphics".
- New gameplay settings: countdown_before_song (wired end-to-end, default off);
  miss_penalty + fail_behavior (persist-only stubs); "Note highway speed"
  surfaces existing master_difficulty.
- New POST /api/settings/reset clears whitelisted keys back to defaults.

Tests: test_settings_api.py, test_plugins.py::test_settings_category_parsed_from_manifest,
tests/browser/settings-tabbed.spec.ts. 179 passed locally.

Ported from the pre-rename feat/v3-settings-tabbed WIP onto current main
(slopsmith→feedBack rename applied; settings-screen markup conflict resolved
in favour of the new tabbed layout — all prior setting ids preserved).

Closes #579

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/__init__.py

@@ -1330,6 +1330,18 @@ def load_plugins(app: FastAPI, context: dict, progress_cb=None, route_setup_fn=N
         _category = manifest.get("category")
         if not isinstance(_category, str) or not _category:
             _category = None
+        # Settings-tab placement (tabbed settings page). When `settings` is a
+        # dict, an optional `category` field names which settings tab the
+        # plugin's panel mounts under (e.g. "graphics", "mic", "progression").
+        # Distinct from the top-level `category` above (which drives Pedalboard
+        # grouping) so the two don't collide. Absent/blank → None → the
+        # frontend falls back to the generic "Plugins" tab.
+        _settings_manifest = manifest.get("settings")
+        _settings_category = None
+        if isinstance(_settings_manifest, dict):
+            _sc = _settings_manifest.get("category")
+            if isinstance(_sc, str) and _sc:
+                _settings_category = _sc
         _icon = manifest.get("icon")
         if not isinstance(_icon, str) or not _icon:
             _icon = None
@@ -1355,6 +1367,7 @@ def load_plugins(app: FastAPI, context: dict, progress_cb=None, route_setup_fn=N
             "has_screen": bool(manifest.get("screen")),
             "has_script": bool(manifest.get("script")),
             "has_settings": bool(manifest.get("settings")),
+            "settings_category": _settings_category,
             "has_tour": _is_valid_tour_manifest(manifest.get("tour")),
             # `styles` is an optional relpath (under the plugin's assets/) to a
             # compiled, preflight-off stylesheet the frontend injects as a
@@ -2067,6 +2080,9 @@ def register_plugin_api(app: FastAPI):
                 "has_screen": p["has_screen"],
                 "has_script": p["has_script"],
                 "has_settings": p["has_settings"],
+                # Settings-tab placement; None when the manifest's `settings`
+                # is absent, a bare string, or omits `category`.
+                "settings_category": p.get("settings_category"),
                 "has_tour": p.get("has_tour", False),
                 # `.get()` fallbacks keep stubbed test entries (built without
                 # _nav_entry) working — styles is None when unset.
@@ -2115,6 +2131,7 @@ def register_plugin_api(app: FastAPI):
                 "has_screen": e.get("has_screen", False),
                 "has_script": e.get("has_script", False),
                 "has_settings": e.get("has_settings", False),
+                "settings_category": e.get("settings_category"),
                 "has_tour": e.get("has_tour", False),
                 "has_styles": e.get("has_styles", False),
                 "styles": e.get("styles"),