Fix issue 7 add dark mode

[SJeffZhang]

Summary

• Fixes 添加黑夜模式 #7
• Add a persistent light/dark theme switcher in the app shell.
• Apply theme state through root-level design tokens instead of page-specific ad hoc classes.
• Update the navigation shell, homepage, detail page, and candidate pool surfaces so dark mode has consistent contrast and glass styling.
• Add an app-shell test that verifies theme toggling and persistence.

Verification

frontend@0.0.0 test:run
vitest run

RUN v4.1.4 /Users/zhangshijie/Desktop/Project/AI_paper_summary_website/frontend

Test Files 6 passed (6)
Tests 18 passed (18)
Start at 00:01:09
Duration 2.95s (transform 2.05s, setup 432ms, import 8.77s, tests 977ms, environment 5.97s)

frontend@0.0.0 build
vite build

vite v8.0.1 building client environment for production...
�[2K
transforming...✓ 1647 modules transformed.
rendering chunks...
computing gzip size...
dist/index.html 0.46 kB │ gzip: 0.29 kB
dist/assets/index-uUTRY9XC.css 387.13 kB │ gzip: 53.84 kB
dist/assets/index-B_hU-7mb.js 1,050.69 kB │ gzip: 341.77 kB

✓ built in 459ms