feat(B.1 #11): versioned settings schema with forward migration

Adds src/lib/utils/settingsMigrations.ts exposing
loadSettingsWithMigration() / saveSettingsWithVersion() around a
{version, data} envelope in localStorage["kon_settings"]. The
migration chain is indexed by destination version so adding a v2
is one MIGRATIONS[2] = (prev) => next entry away from working.

Legacy bare-object settings blobs are treated as v0 and folded into
v1 identically to before — no user-facing reset — but an unreadable
blob now surfaces a single Settings reset toast instead of silently
dropping data.

Covers Handy #602 ('settings reset on update') and unblocks the
other B.2/B.3 SettingsState additions listed in
docs/whisper-ecosystem/workstream-B.md: every subsequent field
lands behind a MIGRATIONS step, so older Kon builds stay readable.

Co-authored-by: jars <jakejars@users.noreply.github.com>

src/lib/stores/page.svelte.ts

@@ -18,6 +18,10 @@ import type {
 import { toasts } from "$lib/stores/toasts.svelte.ts";
 import { errorMessage } from "$lib/utils/errors.js";
 import { parseStoredJson } from "$lib/utils/storage.js";
+import {
+  loadSettingsWithMigration,
+  saveSettingsWithVersion,
+} from "$lib/utils/settingsMigrations.js";
 import { hasTauriRuntime } from "$lib/utils/runtime.js";
 
 export const page = $state<PageState>({
@@ -68,9 +72,25 @@ function canUseStorage(): boolean {
 
 function loadSettings(): SettingsState {
   if (!canUseStorage()) return { ...defaults };
+  // Versioned migration: reads the envelope or an unversioned legacy
+  // blob, runs the migration chain (currently identity: bare object ->
+  // v1), and returns the current-shape data. Falls back to defaults
+  // on corruption — the toast below surfaces the reset to the user.
+  const migrated = loadSettingsWithMigration<Partial<SettingsState>>(SETTINGS_KEY);
+  if (migrated === null && localStorage.getItem(SETTINGS_KEY) !== null) {
+    // There was data but it failed to migrate — tell the user we
+    // reset rather than silently mangling their preferences.
+    queueMicrotask(() => {
+      toasts.warn(
+        "Settings reset",
+        "Your saved settings couldn't be read, so Kon fell back to defaults. "
+          + "Earlier Kon builds can still read your old data.",
+      );
+    });
+  }
   return {
     ...defaults,
-    ...(parseStoredJson<Partial<SettingsState>>(localStorage.getItem(SETTINGS_KEY)) ?? {}),
+    ...(migrated ?? {}),
   };
 }
 
@@ -78,9 +98,11 @@ export const settings = $state<SettingsState>(loadSettings());
 
 export function saveSettings() {
   if (!canUseStorage()) return;
-  try {
-    localStorage.setItem(SETTINGS_KEY, JSON.stringify(settings));
-  } catch {}
+  // Versioned envelope {version, data}. Older Kon builds that used
+  // the bare-object layout can still spread the `data` key over
+  // their defaults — they will just silently drop fields they don't
+  // know about, which is the same behaviour as today.
+  saveSettingsWithVersion(SETTINGS_KEY, { ...settings });
 }
 
 function loadProfiles(): Profile[] {