feat: add all Draw.io themes to settings panel (#835)

* feat: add all Draw.io themes to settings panel

Add all available Draw.io themes (kennedy, atlas, dark, min, sketch, simple)
to the settings panel dropdown. Previously only min and sketch were available
as a toggle button.

Changes:
- Replace the Draw.io style toggle button with a dropdown selector
- Expand theme type from "min" | "sketch" to include all 6 themes
- Update localStorage validation to accept all themes
- Update handler from toggle to direct theme selection

Closes #499

* fix: localize theme labels, tighten DrawioTheme typing, sync dark param

- Move DRAWIO_THEMES + DrawioTheme to lib/drawio-themes.ts; reuse in
  page.tsx, chat-panel.tsx and settings-dialog.tsx instead of `string`
- Localize theme dropdown labels (Dark/Minimal/Sketch/Simple) in
  en/zh/ja/zh-Hant; keep proper-noun themes (Kennedy/Atlas) as-is
- Drop trailing colon from drawioStyleDescription and remove dead
  switchTo/minimal/sketch keys in all 4 dictionaries
- Auto-sync drawio dark URL param when ui="dark" is selected
- Add aria-label to drawio-style SelectTrigger

* fix: use kennedy as default theme and label it "Default"

---------

Co-authored-by: dayuan.jiang <jdy.toh@gmail.com>

lib/drawio-themes.ts

@@ -0,0 +1,17 @@
+export const DRAWIO_THEMES = [
+    "kennedy",
+    "atlas",
+    "dark",
+    "min",
+    "sketch",
+    "simple",
+] as const
+
+export type DrawioTheme = (typeof DRAWIO_THEMES)[number]
+
+export function isDrawioTheme(value: unknown): value is DrawioTheme {
+    return (
+        typeof value === "string" &&
+        (DRAWIO_THEMES as readonly string[]).includes(value)
+    )
+}