White base theme + make Dashboard the default landing page

# Why?

**As a** user of the redesigned web frontend
**I want** a white base color scheme, and to land on the Dashboard by default
**So that** the app matches the lighter look the user actually wants, and so the most useful page (status/alerts/actions) is what greets me on login instead of a secondary page.

# What?

Two small, independent tweaks to the design system/routing introduced in the Phase 1 (#333) restyle and the Phase 3 (#335) dashboard:
- Change the scoped `.app-shell` theme's base/background color to white (currently whatever Phase 1 shipped — check `web/components/ui`/`(protected)/layout.tsx`'s CSS custom properties).
- Make `/dashboard` the default landing route after login instead of `/browse` — likely a redirect change wherever post-login routing currently points (`(protected)/layout.tsx`, or the root `(protected)` index route/middleware), plus updating any Playwright specs that assert on the post-login redirect target (e.g. `golden-path.spec.ts` currently expects `toHaveURL(/\/browse/)`).

# Hard constraints

`/login` and `/test-login` must remain untouched — theme scoping must stay confined to `.app-shell`, not global.

# Testing

Update Playwright specs that assert on post-login URL/landing page.

先檢查 web/components/ui/(protected)/layout.tsx 中的 CSS 自訂屬性，以找出目前的主題背景色。接著，在相同的 layout 檔案中，或在 root (protected) index route/middleware 中找出登入後的路由邏輯，將預設重新導向從 /browse 改為 /dashboard。更新 Playwright 測試 golden-path.spec.ts，使其預期新的 landing URL。執行更新後的 Playwright spec 來驗證變更，確保重新導向正常運作，且白色主題只會套用在 .app-shell scope 內。

由索引模型根據 Issue 內容生成。