feat(pay): real wallet adapter — Privacy Wallet API via get-starknet v6 + WalletAccountV6

Pinned, script-less installs of the official stack (starknet@10.4.0 next-tag
API, get-starknet 6.0.2, types-js 0.10.3). The adapter discovers wallets with
the late-injection rescan pattern, detects STRK20 capability from advertised
Wallet-API versions (never by probing balances), and maps the adapter surface
to exact STRK20_ACTION shapes: deposit / transfer / withdraw, one action per
transaction — a deposit is never bundled with a transfer, per the protocol's
own unlinkability guidance. Heavy libraries load lazily so the widget stays
importable without them.

26 tests green; action shapes pinned by tests against the wallet-api spec.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>