feat(#448): schema-driven config UI with category tabs, search, typed inputs

Closes #448

Replaced the raw YAML editor with a full schema-driven config management UI
matching the dashboard's ConfigPage experience.

### New API endpoints (HermesApiService)
- GET /api/config — load structured config
- GET /api/config/schema — load schema with field types + categories  
- GET /api/config/defaults — load defaults for reset
- PUT /api/config — save config (deep-merged on backend)

### Data models
- ConfigSchemaResponse — wraps fields + category_order
- SchemaField — type, description, category, options
- ConfigUpdateRequest — config map + optional profile

### ConfigViewModel
- Loads config + schema + defaults + path on init
- Tracks pending changes as dot-path → JsonElement map
- Builds nested JSON changeset for the save API
- YAML mode toggle with raw YAML load/save
- Per-category reset to defaults

### ConfigScreen
- Category tabs with field-count badges
- Search filtering across key, label, description, category
- Typed inputs: boolean toggles, select dropdowns, number inputs
- Modified highlighting with primaryContainer tint
- YAML ↔ Form mode toggle
- Sticky save indicator with pending-change count

app/src/main/res/values/strings.xml

@@ -336,7 +336,8 @@
     <string name="config_screen_title">Configuration</string>
     <string name="config_placeholder_yaml">YAML Config...</string>
     <string name="config_action_reset">Reset</string>
-    <string name="config_action_save">Save Config</string>
+    <string name="config_action_save">Save</string>
+    <string name="config_action_save_yaml">Save YAML</string>
 
     <!-- Profiles Screen -->
     <string name="profiles_empty_title">No profiles available</string>