v0.2 Phase 3: additive semantic tokens + KI-05 resolution

Additive token grammar (no renames, no replacements):

- --color-caution (dark #e8be4a, light #a08a1f) becomes the canonical
  name for tuned-amber notice surfaces in the new wrapper grammar
- --color-warning is kept as a CSS var() alias of --color-caution so
  every existing text-warning / bg-warning call site stays valid
- --color-info (dark #7a9ec0, light #3d6a8a) is the soft blue-grey
  signal for the new LumotiaNotice info variant
- --color-accent-environment (dark #8fae9a, light #4a7058) is an
  optional sage/moss support token for empty-state illustrations
  and environment-neutral status dots. NOT a brand swap — amber/
  copper --color-accent stays primary

Mirrored in src/design-system/colors_and_type.css (the buildless
preview pages bypass Tailwind so the duplication is intentional).

KI-05 resolved in the same commit, per the plan:

- src/lib/types/app.ts: drop `theme` from SettingsState
- src/lib/stores/page.svelte.ts: drop `theme: "Dark"` from defaults
- src/routes/+layout.svelte: drop the migration $effect, add a
  one-shot migrateLegacyTheme() on mount that copies any historical
  lumotia_settings.theme into preferences.theme and strips the
  legacy field. Idempotent — subsequent loads short-circuit
- src/routes/{float,viewer,preview}/+layout@.svelte: drop the same
  $effect; secondary windows inherit theme via PREFERENCES_CHANGED_EVENT
- src/lib/pages/SettingsPage.svelte: both SegmentedButton bindings
  (quick-settings row at :1272, Appearance group at :2606) now use
  Svelte 5 function bindings ({ get, set }) backed by prefs.theme
  and updatePreferences. No SegmentedButton API change

Phase 3 per-page gate green: npm run check (0/0/4129 files),
npm test (13/13), npm run test:e2e (16/16). No regressions in
the Phase 1 smoke baseline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/routes/+layout.svelte

@@ -56,16 +56,28 @@
     $sveltePage.url.pathname.startsWith("/viewer")
   );
 
-  // Theme — migrate from old class-based to new data-attribute system
-  // The preferences store handles DOM application via data-theme attribute
-  $effect(() => {
-    // Sync legacy settings.theme → preferences store
-    const legacyTheme = settings.theme;
-    const mapped = legacyTheme === "Light" ? "light" : legacyTheme === "Dark" ? "dark" : "system";
-    if (prefs.theme !== mapped) {
-      updatePreferences({ theme: mapped });
-    }
-  });
+  // KI-05 resolution (v0.2 Phase 3) — preferences.theme is canonical.
+  // The legacy settings.theme field is gone from SettingsState; a
+  // one-shot localStorage migration (migrateLegacyTheme below) runs
+  // once on mount to copy any historical "Dark"/"Light"/"System"
+  // value into preferences, then strips the field so subsequent
+  // loads short-circuit.
+  function migrateLegacyTheme() {
+    if (typeof localStorage === "undefined") return;
+    const raw = localStorage.getItem("lumotia_settings");
+    if (!raw) return;
+    let parsed;
+    try { parsed = JSON.parse(raw); } catch { return; }
+    const data = parsed?.data ?? parsed;
+    if (!data || typeof data !== "object" || !("theme" in data)) return;
+    const legacy = data.theme;
+    const mapped = legacy === "Light" ? "light" : legacy === "System" ? "system" : "dark";
+    if (mapped !== prefs.theme) updatePreferences({ theme: mapped });
+    delete data.theme;
+    if (parsed && typeof parsed === "object" && "data" in parsed) parsed.data = data;
+    else parsed = data;
+    try { localStorage.setItem("lumotia_settings", JSON.stringify(parsed)); } catch {}
+  }
 
   // Global hotkey registration — dual backend
   // Wayland: evdev via lumotia-hotkey crate (works without display server)
@@ -340,6 +352,12 @@
   }
 
   onMount(async () => {
+    // KI-05 v0.2 one-shot: copy any legacy settings.theme into
+    // preferences.theme, then strip it from localStorage. Runs first
+    // so the DOM picks up the right data-theme attribute before any
+    // theme-sensitive rendering.
+    migrateLegacyTheme();
+
     // Auto-collapse if window is already narrow on first load
     handleResize();
     window.addEventListener("resize", handleResize);