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/types.ts

@@ -597,3 +597,12 @@ export interface HermesWriteOutcome {
   backupPath?: string;
   warnings: HermesHealthWarning[];
 }
+
+export type HermesMemoryKind = "memory" | "user";
+
+export interface HermesMemoryLimits {
+  memory: number;
+  user: number;
+  memoryEnabled: boolean;
+  userEnabled: boolean;
+}