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/java/com/m57/hermescontrol/data/model/Config.kt

@@ -1,5 +1,7 @@
 package com.m57.hermescontrol.data.model
 
+import com.google.gson.JsonElement
+
 data class RawConfigResponse(
     val path: String?,
     val yaml: String?,
@@ -9,3 +11,20 @@ data class UpdateRawConfigRequest(
     val yaml_text: String,
     val profile: String? = null,
 )
+
+data class ConfigSchemaResponse(
+    val fields: Map<String, SchemaField>,
+    val category_order: List<String>,
+)
+
+data class SchemaField(
+    val type: String,
+    val description: String? = null,
+    val category: String? = null,
+    val options: List<String>? = null,
+)
+
+data class ConfigUpdateRequest(
+    val config: Map<String, JsonElement>,
+    val profile: String? = null,
+)