feat: add session history with auto-save

- Add Rust backend for session persistence using tauri-plugin-store
- Create SessionHistoryPanel component for browsing saved sessions
- Implement debounced auto-save (2s delay) when messages are added
- Add session search, resume, and delete functionality
- Add clock icon button in StatusBar to access session history

Closes #14

src/lib/components/StatusBar.svelte

@@ -19,6 +19,7 @@
   import HelpPanel from "./HelpPanel.svelte";
   import KeyboardShortcutsModal from "./KeyboardShortcutsModal.svelte";
   import { achievementProgress } from "$lib/stores/achievements";
+  import SessionHistoryPanel from "./SessionHistoryPanel.svelte";
 
   const DISCORD_URL = "https://chat.nhcarrigan.com";
   const DONATE_URL = "https://donate.nhcarrigan.com";
@@ -33,6 +34,7 @@
   let showAbout = $state(false);
   let showHelp = $state(false);
   let showKeyboardShortcuts = $state(false);
+  let showSessionHistory = $state(false);
   const progress = $derived($achievementProgress);
   let currentConfig: HikariConfig = $state({
     model: null,
@@ -217,6 +219,20 @@
         </span>
       {/if}
     </button>
+    <button
+      onclick={() => (showSessionHistory = true)}
+      class="p-1 text-gray-500 hover:text-[var(--accent-primary)] transition-colors"
+      title="Session History"
+    >
+      <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+        <path
+          stroke-linecap="round"
+          stroke-linejoin="round"
+          stroke-width="2"
+          d="M12 8v4l3 3m6-3a9 9 0 11-18 0 9 9 0 0118 0z"
+        />
+      </svg>
+    </button>
     <button
       onclick={() => (showStats = !showStats)}
       class="p-1 text-gray-500 hover:text-[var(--accent-primary)] transition-colors {showStats
@@ -371,3 +387,7 @@
 {#if showKeyboardShortcuts}
   <KeyboardShortcutsModal onClose={() => (showKeyboardShortcuts = false)} />
 {/if}
+
+{#if showSessionHistory}
+  <SessionHistoryPanel onClose={() => (showSessionHistory = false)} />
+{/if}