feat(#363): add descriptive text to loading spinners (#406)

* feat(#363): add descriptive text to loading spinners

- Updates LoadingState composable to accept optional subtitle
- Shows 'Fetching model providers...' on ModelScreen
- Shows 'Loading channels...' on ChannelsScreen
- Removes dead/unreachable CircularProgressIndicator from ModelScreen

* fix(#363): pass loading_state_subtitle_models string to LoadingState

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Co-authored-by: M57 <hy4ri@users.noreply.github.com>

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

@@ -397,4 +397,9 @@
     <string name="landing_subtitle">Control panel for Hermes Agent</string>
     <string name="landing_action_auth_login">Auth Login</string>
     <string name="landing_action_pairing_login">Pairing Login</string>
+
+    <!-- Loading State -->
+    <string name="loading_state_subtitle_default">Loading…</string>
+    <string name="loading_state_subtitle_models">Fetching model providers…</string>
+    <string name="loading_state_subtitle_channels">Loading channels…</string>
 </resources>