feat(hermes): replace Prompts entry with Memory panel

Hermes has no slash-prompt concept (templates live as Skills), so the
Prompts tab for the Hermes app was always empty. Swap the toolbar Book
button for a Brain button that opens a new Memory panel editing
~/.hermes/memories/{MEMORY,USER}.md — Hermes' first-class memory store
which its Web UI exposes only as on/off toggles, never as an editor.

The panel shows each file in its own tab with a character-budget bar
read from config.yaml's nested memory.* section (memory_char_limit /
user_char_limit, default 2200 / 1375). Edits are written atomically;
Hermes picks them up on the next session start per MemoryStore.

Also extract useDarkMode to src/hooks/useDarkMode.ts — the codebase
already repeats the same MutationObserver pattern in 12+ places; this
PR introduces the shared hook and uses it once, leaving the migration
of the other copies to a follow-up.

src/lib/api/hermes.ts

@@ -1,5 +1,10 @@
 import { invoke } from "@tauri-apps/api/core";
-import type { HermesModelConfig, HermesHealthWarning } from "@/types";
+import type {
+  HermesHealthWarning,
+  HermesMemoryKind,
+  HermesMemoryLimits,
+  HermesModelConfig,
+} from "@/types";
 
 /**
  * Hermes Agent configuration API (CC Switch side).
@@ -27,4 +32,25 @@ export const hermesApi = {
   async openWebUI(path?: string): Promise<void> {
     await invoke("open_hermes_web_ui", { path: path ?? null });
   },
+
+  /**
+   * Read one of Hermes' memory blobs (`MEMORY.md` or `USER.md`). Returns an
+   * empty string when the file hasn't been created yet.
+   */
+  async getMemory(kind: HermesMemoryKind): Promise<string> {
+    return await invoke("get_hermes_memory", { kind });
+  },
+
+  /** Atomically overwrite a Hermes memory file. */
+  async setMemory(kind: HermesMemoryKind, content: string): Promise<void> {
+    await invoke("set_hermes_memory", { kind, content });
+  },
+
+  /**
+   * Character budgets + enable flags for both memory blobs, read from
+   * config.yaml with Hermes defaults as fallback.
+   */
+  async getMemoryLimits(): Promise<HermesMemoryLimits> {
+    return await invoke("get_hermes_memory_limits");
+  },
 };