feat: another wave of features (#61)

## Explanation

This PR bundles several user-facing improvements and feature additions for the v0.3.0 release, including quality-of-life improvements to the UI, new slash commands, better state persistence, and auto-update checking.

## Included Changes

- **Resizable chat input** with drag handle (#58 partial)
- **Arrow key navigation fix** - cursor keys now navigate text when user has typed input (#58)
- **Scroll position persistence** per conversation tab
- **/skill command** for invoking Claude Code skills (#57)
- **Stats persistence fix** - stats now persist across session changes, only reset on disconnect (#59)
- **Auto-update checker** on startup (#17)
- **Resizable character panel** with full-height sprites (#10)
- **Font size and zoom settings** with keyboard shortcuts (Ctrl++/Ctrl+-/Ctrl+0) (#19)

## Closes

Closes #10, #17, #19, #57, #58, #59

## Attestations

- [x] I have read and agree to the Code of Conduct
- [x] I have read and agree to the Community Guidelines
- [x] My contribution complies with the Contributor Covenant
- [x] I have run the linter and resolved any errors
- [x] My pull request uses an appropriate title, matching the conventional commit standards
- [x] My scope of feat/fix/chore/etc. correctly matches the nature of changes in my pull request
- [x] All new and existing tests pass locally with my changes
- [x] Code coverage remains at or above the configured threshold

## Documentation

N/A - Internal app features

## Versioning

Minor - My pull request introduces new non-breaking features.

---
✨ This PR was created with help from Hikari~ 🌸

Co-authored-by: Hikari <hikari@nhcarrigan.com>
Reviewed-on: #61
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

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}