feat(wallet): subscribe to Freighter network changes, poll 5s fallback, disable Execute on wrong network (#42)

[collinsezedike]

Summary

• useFreighter: replaced 300ms polling with WatchWalletChanges subscription (5s interval); falls back to 5s setInterval polling if the API is unavailable
• WalletContext: same subscription pattern via lazy dynamic import (avoids SSR failures from a static top-level import); single cleanup covers both paths
• WalletButton: consumes useWallet from context so the wrong-network badge re-renders reactively on network switch
• RateTable: accepts executeDisabled prop; Execute button is disabled whenever the app network is not PUBLIC

Test plan

• Switch Freighter to Testnet — WalletButton shows "Wrong network / Mainnet required" within 5s
• Execute buttons in the rate table become disabled on wrong network
• Switch back to Mainnet — WalletButton returns to connected state and Execute re-enables
• With Freighter not installed, confirm no SSR crash and the "Install Freighter" link renders

Closes #42

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[ezedike-evan]

Code quality is solid — the lazy dynamic import of `WatchWalletChanges` correctly eliminates the SSR crash, the 5s polling fallback is clean with proper cleanup, and `executeDisabled={network !== 'PUBLIC'}` in the rate table is the right place for that guard. The test mock upgrade in `useFreighter.spec.tsx` is a real improvement over the stub.

One thing to fix before this can land: this branch has a merge conflict with `main`. Please rebase on `main` and resolve any conflicts, then push — I'll merge immediately after.

```bash
git fetch origin
git rebase origin/main

resolve any conflicts, then:

git push --force-with-lease
```

Note: PR #262 (mid-session install detection) is stacked on top of this branch. Rebase this one first, then rebase #262 on top of the updated branch.

[ezedike-evan]

Approved and ready to land. Sole remaining blocker is the merge conflict — please rebase and push:

```bash
git fetch origin
git rebase origin/main

resolve any conflicts, then:

git push --force-with-lease
```

Once this is merged, ping here so I can trigger the rebase on #262 (which is stacked on top of this branch).