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-tauri/src/commands/hermes.rs

@@ -57,6 +57,25 @@ pub fn get_hermes_model_config() -> Result<Option<hermes_config::HermesModelConf
     hermes_config::get_model_config().map_err(|e| e.to_string())
 }
 
+// ============================================================================
+// Memory Files Commands
+// ============================================================================
+
+#[tauri::command]
+pub fn get_hermes_memory(kind: hermes_config::MemoryKind) -> Result<String, String> {
+    hermes_config::read_memory(kind).map_err(|e| e.to_string())
+}
+
+#[tauri::command]
+pub fn set_hermes_memory(kind: hermes_config::MemoryKind, content: String) -> Result<(), String> {
+    hermes_config::write_memory(kind, &content).map_err(|e| e.to_string())
+}
+
+#[tauri::command]
+pub fn get_hermes_memory_limits() -> Result<hermes_config::HermesMemoryLimits, String> {
+    hermes_config::read_memory_limits().map_err(|e| e.to_string())
+}
+
 // ============================================================================
 // Hermes Web UI launcher
 // ============================================================================