feat: initial Tatsumi release

Tatsumi is a Tauri desktop app for generating AI character art of Naomi
using Google Gemini's image model. Features three generation modes
(avatar, art, replace), persistent conversation threads, message
editing and deletion, retry support, cost tracking, and an about modal
with lore-accurate self-introduction from Emi Carrigan.

src/types/index.ts

@@ -0,0 +1,44 @@
+/**
+ * @file Type definitions for the Tatsumi application.
+ * @copyright Naomi Carrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+
+type Mode = "avatar" | "art" | "replace";
+
+interface Config {
+  apiKey: string;
+}
+
+interface PendingInput {
+  imageBase64?:  string;
+  imageMime?:    string;
+  imagePreview?: string;
+  text?:         string;
+}
+
+interface MessagePart {
+  imageData?:        string;
+  mimeType?:         string;
+  text?:             string;
+  thoughtSignature?: string;
+  type:              "image" | "text" | "thought";
+}
+
+interface ThreadMessage {
+  cost?: number;
+  parts: Array<MessagePart>;
+  role:  "user" | "model";
+}
+
+interface Thread {
+  createdAt: number;
+  id:        string;
+  messages:  Array<ThreadMessage>;
+  mode:      Mode;
+  name:      string;
+  updatedAt: number;
+}
+
+export type { Config, MessagePart, Mode, PendingInput, Thread, ThreadMessage };