feat: add user-selectable aspect ratio and resolution per thread (#18)

## Summary

- Adds a two-step thread creation modal — step 1 picks the mode, step 2 configures generation options
- Art mode now supports user-selectable aspect ratio (1:1, 4:3, 3:4, 16:9, 9:16, 21:9)
- All three modes (Art, Avatar, Replace) now support user-selectable resolution (1K, 2K, 4K)
- Mode label in the input area reflects the chosen settings (e.g. `🩷 Art Mode (16:9) · 4K`)
- Backend cost calculation now scales with resolution
- Regenerated `icon.ico` with clean BMP-only entries to fix Windows local builds

✨ This PR was created with help from Hikari~ 🌸

Reviewed-on: #18
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

src-tauri/src/storage.rs

@@ -25,11 +25,19 @@ pub struct ThreadMessage {
     pub cost_usd: Option<f64>,
 }
 
+fn default_image_size() -> String {
+    "4K".to_string()
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Thread {
     pub id: String,
     pub name: String,
     pub mode: String,
+    #[serde(rename = "aspectRatio", skip_serializing_if = "Option::is_none")]
+    pub aspect_ratio: Option<String>,
+    #[serde(rename = "imageSize", default = "default_image_size")]
+    pub image_size: String,
     pub messages: Vec<ThreadMessage>,
     #[serde(rename = "createdAt")]
     pub created_at: i64,