feat: add ability to configure the agent (also theme switcher) (#3)

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### Tests

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Reviewed-on: #3
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

src-tauri/Cargo.lock

@@ -1403,6 +1403,8 @@ dependencies = [
  "tauri-plugin-dialog",
  "tauri-plugin-opener",
  "tauri-plugin-shell",
+ "tauri-plugin-store",
+ "tempfile",
  "tokio",
  "uuid",
 ]
@@ -3761,6 +3763,22 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "tauri-plugin-store"
+version = "2.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5ca1a8ff83c269b115e98726ffc13f9e548a10161544a92ad121d6d0a96e16ea"
+dependencies = [
+ "dunce",
+ "serde",
+ "serde_json",
+ "tauri",
+ "tauri-plugin",
+ "thiserror 2.0.17",
+ "tokio",
+ "tracing",
+]
+
 [[package]]
 name = "tauri-runtime"
 version = "2.9.2"