[3/7] React migration: Header, Footer and Settings panel

[devanshi-gpta]

Summary

Ports the app chrome from src/app/core/ and mounts it in App. Header renders the settings panel itself when settings.showSettings is true, exactly as the Angular template did with *ngIf.

• routerLink + routerLinkActive → NavLink with an isActive class callback; the scroll-to-top click handler is preserved.
• Settings translates the theme radios, font-size and spacing selects and the new-tab checkbox into controlled inputs driven by useSettings().
• Header image src values are root-relative (/assets/images/logo.svg). Angular resolved the relative form via <base href="/">, which Vite's index.html has no equivalent for — left relative, the logo and cog 404 on any nested route such as /item/123.
• Stylesheets are wrapped in .c-header / .c-footer / .c-settings to replace Angular's emulated encapsulation.

Proposed fixes

• Add Header.tsx, Footer.tsx, Settings.tsx and their stylesheets.
• Render Header and Footer in the App layout.

Link to Devin session: https://app.devin.ai/sessions/a3a0ff57bab64e25860349a003a282ef
Open in Devin Desktop: https://app.devin.ai/desktop/session/a3a0ff57bab64e25860349a003a282ef?variant=devin
Requested by: @devanshi-gpta

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