feature: PR 3/8 — SettingsContext and SCSS theming

[devin-ai-integration]

Summary

PR 3 of 8 (stacked on #699). Replaces SettingsService with a React context and brings the SCSS theme layer into the React tree. src/app/ and src/styles.scss are untouched — Angular still builds from them.

• src/react/context/SettingsContext.tsx — SettingsProvider + useSettings(). Same localStorage keys (theme, openLinkInNewTab, titleFontSize, listSpacing), same defaults ('16' / '0'), same string types, same write-through-on-set behavior, and the same prefers-color-scheme handling: use the stored theme if present, otherwise matches ? 'night' : 'default' (read directly instead of Angular's synthetic MediaQueryListEvent dispatch).
• A system scheme change still overwrites (and persists) the user's chosen theme — that's the existing app behavior, kept deliberately.
• The listener is registered in useEffect and torn down with the same function reference. The Angular service passed a fresh .bind(this) to removeEventListener, so its listener was never actually removed; not reproduced.
• localStorage / matchMedia access is guarded so the module can't throw where they're unavailable.
• SCSS: sass added as a devDependency; styles.scss and the three partials copied to src/react/ byte-identically except the partial import path, and imported from main.tsx. Sass prints legacy @import/slash-division/darken() deprecation warnings inherited from the original files.
• vite.config.ts: the entry-injecting transformIndexHtml hook now runs with order: 'pre', otherwise the injected module script isn't part of the build graph and the SCSS never reaches the output.
• App temporarily applies settings.theme as its wrapper className; PR 4 replaces it with the real shell.

Verified: npm run typecheck:react, npx vite build (built CSS contains .default / .night / .amoledblack), and vite dev server boots clean.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/e6419ebe67ff4f749fe1573a8e26cd07
Requested by: @abboudp

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