feat(hermes): memory enable switch + clearer migration warning copy

Replaces the greyed-out "Memory is disabled" banner with a real Switch
at the top of each memory tab. Users can now toggle Hermes' memory/user
blobs without leaving CC Switch; the underlying write goes through the
merge-aware `set_memory_enabled`, so budgets and external-provider
settings survive toggle operations. The new `useToggleHermesMemoryEnabled`
mutation invalidates the limits query so the Switch state and the
amber disabled-hint update in lockstep.

Reworks the `schema_migrated_v12` health banner copy to match the
simplified "CC Switch only manages custom_providers" posture — it now
tells users to reconcile migrated dict entries via Hermes Web UI,
instead of the earlier (and now inaccurate) "CC Switch reads both".

src/lib/api/hermes.ts

@@ -53,4 +53,15 @@ export const hermesApi = {
   async getMemoryLimits(): Promise<HermesMemoryLimits> {
     return await invoke("get_hermes_memory_limits");
   },
+
+  /**
+   * Toggle the on/off flag for one memory blob. Other fields in the `memory:`
+   * section (budgets, external provider config) are preserved.
+   */
+  async setMemoryEnabled(
+    kind: HermesMemoryKind,
+    enabled: boolean,
+  ): Promise<void> {
+    await invoke("set_hermes_memory_enabled", { kind, enabled });
+  },
 };