fix(vite): drop deprecated swc/tsconfig-paths plugins, use oxc/rolldown options

[EinfachValle]

Summary

Silences the four Vite 8 / Rolldown deprecation warnings that show up on every yarn dev:

• vite-tsconfig-paths plugin → native resolve.tsconfigPaths: true (drops the dep)
• @vitejs/plugin-react-swc → @vitejs/plugin-react (recommended now that we use no SWC plugins; React Compiler / Babel peers are optional)
• optimizeDeps.esbuildOptions → optimizeDeps.rolldownOptions.transform
• top-level esbuild.target → oxc.target (Vite 8's transform pipeline is Oxc, not esbuild — esbuild options are silently ignored)

Applied to app/vite.config.ts, app/vitest.config.ts, landingpage/vite.config.ts, landingpage/vitest.config.ts.

Test plan

• yarn install clean
• yarn typecheck green
• yarn lint green
• yarn test green (157 + 2 + 6 = 165 files)
• yarn dev:web boots without any of the four warnings; dashboard renders, no new console errors