feat: add auto-update checker

Implements issue #17 - the app now checks for updates on startup and shows
a notification when a newer version is available. Users can disable this
in settings. Uses Gitea releases API with semver comparison.

Closes #17

src/lib/components/ConfigSidebar.svelte

@@ -15,6 +15,7 @@
     notifications_enabled: true,
     notification_volume: 0.7,
     always_on_top: false,
+    update_checks_enabled: true,
   });
 
   let isOpen = $state(false);
@@ -427,6 +428,21 @@
           Keep the window above other windows
         </p>
       </div>
+
+      <!-- Update Checks Toggle -->
+      <div class="mb-4">
+        <label class="flex items-center gap-3 cursor-pointer">
+          <input
+            type="checkbox"
+            bind:checked={config.update_checks_enabled}
+            class="w-4 h-4 text-[var(--accent-primary)] bg-[var(--bg-primary)] border-[var(--border-color)] rounded focus:ring-[var(--accent-primary)] focus:ring-2"
+          />
+          <span class="text-sm text-[var(--text-primary)]">Check for updates on startup</span>
+        </label>
+        <p class="text-xs text-[var(--text-tertiary)] mt-1 ml-7">
+          Notify when a new version is available
+        </p>
+      </div>
     </section>
 
     <!-- Notifications Section -->

src/lib/components/InputBar.svelte

@@ -365,11 +365,7 @@ User: ${formattedMessage}`;
         onSelect={selectCommand}
       />
       <!-- svelte-ignore a11y_no_static_element_interactions -->
-      <div
-        class="resize-handle"
-        onmousedown={handleResizeStart}
-        title="Drag to resize"
-      ></div>
+      <div class="resize-handle" onmousedown={handleResizeStart} title="Drag to resize"></div>
       <textarea
         bind:value={inputValue}
         onkeydown={handleKeyDown}

src/lib/components/StatusBar.svelte

@@ -46,6 +46,7 @@
     notifications_enabled: true,
     notification_volume: 0.5,
     always_on_top: false,
+    update_checks_enabled: true,
   });
 
   onMount(async () => {

src/lib/components/UpdateNotification.svelte

@@ -0,0 +1,92 @@
+<script lang="ts">
+  import { invoke } from "@tauri-apps/api/core";
+  import { openUrl } from "@tauri-apps/plugin-opener";
+  import type { UpdateInfo } from "$lib/types/messages";
+  import { configStore } from "$lib/stores/config";
+
+  let updateInfo = $state<UpdateInfo | null>(null);
+  let dismissed = $state(false);
+
+  export async function checkForUpdates() {
+    // Check if update checks are enabled
+    const config = configStore.getConfig();
+    if (!config.update_checks_enabled) {
+      return;
+    }
+
+    try {
+      const info = await invoke<UpdateInfo>("check_for_updates");
+      if (info.has_update) {
+        updateInfo = info;
+        dismissed = false;
+      }
+    } catch (err) {
+      const errorMessage = err instanceof Error ? err.message : String(err);
+      console.error("Failed to check for updates:", errorMessage);
+    }
+  }
+
+  function dismiss() {
+    dismissed = true;
+  }
+
+  async function openRelease() {
+    if (updateInfo?.release_url) {
+      await openUrl(updateInfo.release_url);
+    }
+  }
+</script>
+
+{#if updateInfo && !dismissed}
+  <div
+    class="fixed bottom-4 right-4 max-w-sm bg-[var(--bg-tertiary)] border border-[var(--accent-primary)] rounded-lg shadow-lg p-4 z-50"
+  >
+    <div class="flex items-start gap-3">
+      <div class="text-2xl">🎉</div>
+      <div class="flex-1">
+        <h3 class="text-[var(--text-primary)] font-semibold mb-1">Update Available!</h3>
+        <p class="text-[var(--text-secondary)] text-sm mb-2">
+          A new version of Hikari Desktop is available:
+          <span class="text-[var(--accent-primary)] font-mono">{updateInfo.latest_version}</span>
+        </p>
+        <p class="text-[var(--text-muted)] text-xs mb-3">
+          Current version: {updateInfo.current_version}
+        </p>
+        <div class="flex gap-2">
+          <button
+            onclick={openRelease}
+            class="px-3 py-1.5 bg-[var(--accent-primary)] text-white rounded text-sm hover:brightness-110 transition-all"
+          >
+            View Release
+          </button>
+          <button
+            onclick={dismiss}
+            class="px-3 py-1.5 bg-[var(--bg-secondary)] text-[var(--text-secondary)] rounded text-sm hover:bg-[var(--bg-primary)] transition-all"
+          >
+            Later
+          </button>
+        </div>
+      </div>
+      <button
+        onclick={dismiss}
+        class="text-[var(--text-muted)] hover:text-[var(--text-primary)] transition-colors"
+        aria-label="Dismiss"
+      >
+        <svg
+          class="w-4 h-4"
+          fill="none"
+          stroke="currentColor"
+          viewBox="0 0 24 24"
+          xmlns="http://www.w3.org/2000/svg"
+        >
+          <path
+            stroke-linecap="round"
+            stroke-linejoin="round"
+            stroke-width="2"
+            d="M6 18L18 6M6 6l12 12"
+          ></path>
+        </svg>
+      </button>
+    </div>
+  </div>
+{/if}