feat: setup-accounts + setup-accounts-extension scripts

[AquiGorka]

Summary

Adds two opt-in account-funding scripts to the local-dev flow so the manual browser-wallet test cycle doesn't require clicking "Fund via Friendbot" inside each extension after every `down → up`.

`setup-accounts.sh` + `lifecycle/setup-accounts.ts` (generic)

Takes one or more Stellar public keys as args and fires Friendbot funding requests in parallel. Idempotent — accounts already funded are reported as such (matches "already funded", "already_exist", and "op_already_exists" wordings since friendbot uses different bodies on quickstart vs testnet).

Exports `fundAccounts` + `formatResults` so other scripts can delegate to it instead of duplicating Friendbot logic.

```
./setup-accounts.sh GABC... GDEF... GHIJ...
```

`setup-accounts-extension.sh` + `lifecycle/setup-accounts-extension.ts` (wallet-aware)

Reads `../browser-wallet/.env.seed.user1` and `.env.seed.user2` (override via `SEED_FILES` env), parses each `SEED_MNEMONIC`, derives the Stellar account at index 0 using SLIP-0010 (matches `Keys.deriveStellarAccountFromMnemonic` in `browser-wallet/src/keys/keys.ts`), then delegates to `setup-accounts`' `fundAccounts`. Eliminates the manual copy-paste of Stellar pubkeys from the wallet seed files.

The SLIP-0010 derivation is replicated inline (we can't import from browser-wallet across repos with separate import maps). Verified to match the wallet's derivation by deriving the same pubkeys from the same mnemonics:

```
.env.seed.user1 → GAPMIPHUED74EX25HS6747PY4ZQCNM6PPBVAXHFDH2M66LE5PZ7LLIDU
.env.seed.user2 → GBRRMXSTA6OWPZYMSJARBSLLHQVGRMO7EGA63ASOSWKDJNEQU73JKZTP
```

Why

Without this, after every `down → up` cycle the user has to:

1. Open each extension
2. Find the "Fund via Friendbot" button
3. Click it
4. Refresh chain state

With this, the typical manual test cycle (combined with #48 deterministic stack) becomes one chain:

```bash
./down.sh && ./up.sh && ./setup-c.sh && ./setup-pp.sh && ./setup-accounts-extension.sh
```

Then just click reload on both extensions and everything is ready.

Test plan

• `./setup-accounts.sh G...` funds an arbitrary pubkey
• Re-running on the same pubkey reports "already funded" without erroring
• `./setup-accounts-extension.sh` reads both wallet seed files, derives correct Stellar pubkeys, funds both via Friendbot
• Idempotent re-run reports both as already funded
• `deno check setup-accounts.ts setup-accounts-extension.ts` passes