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.

Start by examining the CSS custom properties in web/components/ui/(protected)/layout.tsx to locate the current theme background color. Then, find the post-login routing logic in the same layout file or in the root (protected) index route/middleware to change the default redirect from /browse to /dashboard. Update the golden-path.spec.ts Playwright test to expect the new landing URL. Verify the changes by running the updated Playwright spec to ensure the redirect works and the white theme is applied only within the .app-shell scope.

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