feat(wallet): add Freighter + LOBSTR wallet picker modal

[chidii]

Closes #374 (draft — see caveats below, needs verification before ready for review)

Restricts the wallet connection kit to exactly Freighter + LOBSTR per
MVP scope (no Albedo/xBull/Hana). Uses @creit.tech/stellar-wallets-kit
v2's static API (init/createButton/getAddress/signTransaction/on),
verified against the package's current npm listing (v2.0.1) and official
docs at stellarwalletskit.dev.

LOBSTR uses its own dedicated module (not the generic WalletConnect
one), so no WalletConnect Cloud project ID is required.

The kit's own createButton() is used for both connecting (opens its
picker modal, showing only the two configured wallets) and disconnecting
(via its profile modal) — no custom disconnect logic was written, because
no standalone disconnect() method is documented anywhere in the kit's
docs, GitHub, or JSR listing. Routing through the kit's own UI was the
only way to satisfy the disconnect acceptance criterion without guessing
at an undocumented API for wallet-signing code.

Reconnect-on-reload relies on the kit's own internal session persistence
(documented behavior, not something this PR implements manually).

IMPORTANT — nothing in this PR has been run or clicked through by
anyone. Needs, before this leaves draft:

• npm install actually succeeds and the import specifier resolves
  (@creit.tech/stellar-wallets-kit/sdk etc. — if this 404s on build,
  try the hyphenated @creit-tech/stellar-wallets-kit form instead,
  which is what the package's own README shows, inconsistently with
  its registered npm scope name)
• Manual click-through of all four acceptance criteria: Freighter
  connect, LOBSTR connect (needs the LOBSTR mobile app for real QR
  pairing), refresh-doesn't-reprompt, disconnect clears state,
  cancel-leaves-null-no-toast
• A look at frontend/lib/stellar-transaction.ts's SignTransaction type
  to confirm the signTransaction signature here still matches what
  wallet-setup.ts expects