feat(frontend): wire theme, react-query and shared session foundation

[Muhammadjazuli]

Summary

Wires up the foundational client-side tooling that is already installed as dependencies but never configured, fixes broken asset references, and gives session state a single shared home.

Icon assets (FE-94)

• Added the missing public icon files (apple-touch-icon.png, favicon-32x32.png, favicon-16x16.png) so every icon URL declared in layout.tsx metadata now resolves to a real file.

Theme provider (FE-95)

• Wrapped the app in next-themes' ThemeProvider (light/dark/system, persisted preference) with an attribute="class" setup that works with Tailwind's dark: variant.

react-query (FE-96)

• Added a QueryClient + QueryClientProvider and mounted it in layout.tsx.
• Rewrote WalletStatusCard to fetch wallet status with useQuery and to drive every action (provision, link challenge, link verify) through useMutation, removing the hand-rolled loading/error/busy state.

Shared session store (FE-97)

• Added a zustand store (lib/stores/session-store.ts) as the single source of truth for the authenticated session.
• Migrated wallet/page.tsx to read the access token from the shared store instead of Cookies.get directly, and added a sign-out action that clears it in one place.

Verification

• npx tsc --noEmit passes.
• npx eslint passes.

Closes #1623
Closes #1624
Closes #1625
Closes #1626

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