Settings: remove dead 'SHONAR server URL' field; mark sync prefs as pending provider

- server_url setting deleted (no code referenced it; it predated the
  provider pivot and misled users into pasting Nextcloud URLs into a
  field nothing reads). The provider-selection flow (P2) replaces it.
- Wi-Fi-only / charging-only uploads now carry a description stating
  they apply once a sync provider is connected.

Verified: 47 unit tests green, APK builds, on-device (Pixel 8 Pro)
Settings > Network shows no SHONAR server URL field and the new
descriptions render.

android/app/src/main/java/com/shonar/settings/BuiltInSettings.kt

@@ -56,17 +56,10 @@ object BuiltInSettings {
             defaultJson = "\"#4FD1C5\"",
         ),
         // --- Network -------------------------------------------------------------
-        SettingDefinition(
-            id = "server_url",
-            name = "SHONAR server URL",
-            description = "Your self-hosted SHONAR backend.",
-            category = CAT_NETWORK,
-            type = SettingType.URL,
-            defaultJson = "\"\"",
-        ),
         SettingDefinition(
             id = "wifi_only_upload",
             name = "Upload on Wi-Fi only",
+            description = "Applies once a sync provider is connected (next milestone).",
             category = CAT_NETWORK,
             type = SettingType.BOOLEAN,
             defaultJson = "true",
@@ -74,6 +67,7 @@ object BuiltInSettings {
         SettingDefinition(
             id = "charging_only_upload",
             name = "Upload only while charging",
+            description = "Applies once a sync provider is connected (next milestone).",
             category = CAT_NETWORK,
             type = SettingType.BOOLEAN,
             defaultJson = "false",