feat: new drafts feature and sound spam fix (#174)

## Summary

- **Saved Drafts feature**: Users can now save input content as drafts for later use, and manage them from a new panel
- **Sound spam fix**: The "Working on it!" sound no longer plays repeatedly when Claude makes multiple tool calls in a row

## Details

### Drafts feature
- Rust backend (`drafts.rs`) with `list_drafts`, `save_draft`, `delete_draft`, and `delete_all_drafts` commands, persisted to `hikari-drafts.json` via the Tauri Store plugin
- `draftsStore` wrapping all four commands with timestamp formatting
- `DraftPanel` overlay with insert, per-item two-step delete confirmation, delete-all with confirmation, empty state, and slide-in animation
- **Drafts** button in the top control row (pencil icon)
- **Save as Draft** floppy-disk icon button in the button wrapper (disabled when input is empty)

### Sound spam fix
- Root cause: `resetSoundState` was called on **every** `thinking` state transition, including mid-task transitions (`coding → thinking → coding`)
- Fix: only reset sound state when entering `thinking` from a clean-slate state (`idle`, `success`, or `error`) — states that genuinely mark the end of one task and the start of a new one

## Test plan
- [ ] Save a draft and verify it persists across app restarts
- [ ] Insert a draft and verify it populates the input
- [ ] Delete individual drafts and verify delete-all works
- [ ] Verify "Working on it!" plays once per user message regardless of how many tools are called

✨ This PR was created with help from Hikari~ 🌸

Reviewed-on: #174
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

src/lib/stores/drafts.ts

@@ -0,0 +1,87 @@
+import { writable } from "svelte/store";
+import { invoke } from "@tauri-apps/api/core";
+
+export interface Draft {
+  id: string;
+  content: string;
+  saved_at: string;
+}
+
+function createDraftsStore() {
+  const drafts = writable<Draft[]>([]);
+  const isLoading = writable(false);
+
+  async function loadDrafts(): Promise<void> {
+    isLoading.set(true);
+    try {
+      const list = await invoke<Draft[]>("list_drafts");
+      drafts.set(list);
+    } catch (error) {
+      console.error("Failed to load drafts:", error);
+    } finally {
+      isLoading.set(false);
+    }
+  }
+
+  async function saveDraft(content: string): Promise<Draft | null> {
+    try {
+      const draft = await invoke<Draft>("save_draft", { content });
+      await loadDrafts();
+      return draft;
+    } catch (error) {
+      console.error("Failed to save draft:", error);
+      return null;
+    }
+  }
+
+  async function deleteDraft(draftId: string): Promise<boolean> {
+    try {
+      await invoke("delete_draft", { draftId });
+      await loadDrafts();
+      return true;
+    } catch (error) {
+      console.error("Failed to delete draft:", error);
+      return false;
+    }
+  }
+
+  async function deleteAllDrafts(): Promise<boolean> {
+    try {
+      await invoke("delete_all_drafts");
+      drafts.set([]);
+      return true;
+    } catch (error) {
+      console.error("Failed to delete all drafts:", error);
+      return false;
+    }
+  }
+
+  function formatTimestamp(isoString: string): string {
+    const date = new Date(isoString);
+    if (isNaN(date.getTime())) {
+      return isoString;
+    }
+    try {
+      return date.toLocaleString(undefined, {
+        month: "short",
+        day: "numeric",
+        hour: "numeric",
+        minute: "2-digit",
+      });
+    } catch {
+      return isoString;
+    }
+  }
+
+  return {
+    drafts: { subscribe: drafts.subscribe },
+    isLoading: { subscribe: isLoading.subscribe },
+    loadDrafts,
+    saveDraft,
+    deleteDraft,
+    deleteAllDrafts,
+    formatTimestamp,
+  };
+}
+
+export const draftsStore = createDraftsStore();