feat(#442): categorized env keys screen with add/remove, search, and gateway restart (#464)

* feat(#442): categorized env keys screen with add/remove, search, and gateway restart

- Group env vars by category (LLM Providers, Tool API Keys, Messaging
  Platforms, Agent Settings, Other) with collapsible AnimatedVisibility
  sections matching MCP screen pattern
- Add new env var form (PUT /api/env) with key/value inputs
- Delete env var via DELETE /api/env with EnvVarDeleteRequest body
- Reveal/hide/edit inline per-card controls
- Search bar filtering across key names and descriptions
- Gateway restart banner with one-click action when changes are dirty
- Track keysChanged flag showing restart prompt after mutations
- Wire delete endpoint in HermesApiService using @HTTP(hasBody=true)
- Add EnvVarDeleteRequest data model class

* fix(#442): update test to match new categorized KeysViewModel API

* fix(#442): satisfy ktlint multiline-expression-wrapping rule

---------

Co-authored-by: Hermes Agent <hermes@agent.local>

app/src/main/java/com/m57/hermescontrol/data/model/EnvVarConfig.kt

@@ -26,3 +26,8 @@ data class EnvVarUpdate(
     val value: String,
     val profile: String? = null,
 )
+
+data class EnvVarDeleteRequest(
+    val key: String,
+    val profile: String? = null,
+)