feat: dashboard theme system with live switching

Add a theme engine for the web dashboard that mirrors the CLI skin
engine philosophy — pure data, no code changes needed for new themes.

Frontend:
- ThemeProvider context that loads active theme from backend on mount
  and applies CSS variable overrides to document.documentElement
- ThemeSwitcher dropdown component in the header (next to language
  switcher) with instant preview on click
- 6 built-in themes: Hermes Teal (default), Midnight, Ember, Mono,
  Cyberpunk, Rosé — each defines all 21 color tokens + overlay settings
- Theme types, presets, and context in web/src/themes/

Backend:
- GET /api/dashboard/themes — returns available themes + active name
- PUT /api/dashboard/theme — persists selection to config.yaml
- User custom themes discoverable from ~/.hermes/dashboard-themes/*.yaml
- Theme list endpoint added to public API paths (no auth needed)

Config:
- dashboard.theme key in DEFAULT_CONFIG (default: 'default')
- Schema override for select dropdown in config page
- Category merged into 'display' tab in config UI

i18n: theme switcher strings added for en + zh.

hermes_cli/config.py

@@ -559,6 +559,11 @@ DEFAULT_CONFIG = {
         "platforms": {},  # Per-platform display overrides: {"telegram": {"tool_progress": "all"}, "slack": {"tool_progress": "off"}}
     },
 
+    # Web dashboard settings
+    "dashboard": {
+        "theme": "default",  # Dashboard visual theme: "default", "midnight", "ember", "mono", "cyberpunk", "rose"
+    },
+
     # Privacy settings
     "privacy": {
         "redact_pii": False,  # When True, hash user IDs and strip phone numbers from LLM context