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/lib.rs

@@ -1258,6 +1258,9 @@ pub fn run() {
             commands::scan_hermes_config_health,
             commands::get_hermes_model_config,
             commands::open_hermes_web_ui,
+            commands::get_hermes_memory,
+            commands::set_hermes_memory,
+            commands::get_hermes_memory_limits,
             // Global upstream proxy
             commands::get_global_proxy_url,
             commands::set_global_proxy_url,