fix(flutter): show 'wallet connector could not load' instead of 'install Freighter' when the Freighter bridge fails to load

[XxHugheadxX]

Refs #116

Summary

The Freighter bridge (web/freighter_bridge.js) imports @stellar/freighter-api from a CDN (esm.sh). When that import fails, web/index.html swallowed the error, and connecting showed "Freighter not found, install the extension", even to a user who has Freighter installed. The import fails when the user is offline, the CDN is blocked or down, or an ad blocker or network filter stops it. The user was then sent to reinstall a wallet they already have.

This PR tells the two cases apart. The root fix, bundling the Freighter API locally behind a CSP, stays in #116.

• web/index.html sets window.puls3FreighterBridgeFailed = true when the bridge import fails. It still logs the error as before.

• The web bridge (freighter_bridge_web.dart) reads that flag when the bridge functions are missing:

  • the load failed → the new WalletConnectorUnavailable;
  • no flag → WalletNotInstalled, as before.

  The decision is the pure function bridgeMissingError, so it can be unit tested without a browser.

• Messages:

  Where	Shows
  Wallet panel (S09)	"Could not load the wallet connector", the reason (check your connection or ad blocker, then reload) and Try again. There is no Install button
  Hire flow	The same reason, ending in "No funds moved"
  Deploy flow	Maps it to its "wallet unavailable" error

Acceptance criteria

This PR does not close #116. Its criteria (bundle the API, add a CSP) are untouched.

• A bridge that failed to load never tells the user to install Freighter.
• A missing wallet with a loaded bridge still shows "not found" and the Install button.
• Every switch over WalletException handles the new case. It is a sealed class, so the analyzer enforces this.

Verification evidence

$ cd puls3_flutter && flutter analyze --fatal-infos
No issues found!
$ flutter test
00:29 +236: All tests passed!

New tests:

• test/wallet/bridge_missing_error_test.dart:
  • a failed load maps to WalletConnectorUnavailable;
  • no flag still maps to WalletNotInstalled;
  • the hire message never says "Install" and says "No funds moved".
• test/wallet/wallet_connection_test.dart: the panel shows the connector state with Try again and no Install Freighter button.

To check it by hand in Chrome with Freighter installed:

1. Block esm.sh in DevTools → Network request blocking.
2. Reload and click Connect Freighter.
3. Before: "Freighter not found". After: "Could not load the wallet connector", with no Install button.

Notes for reviewers

• Found during the same check, not changed here: the app has no SEP-10 sign-in yet.
  • The server requires a wallet session since feat(server): wallet-authenticated SEP-10 sessions #147, so a real hire currently ends in "Sign-in with your wallet is not available yet".
  • FreighterWallet.signTransaction would also refuse a SEP-10 challenge as it is: _assertSourceBinding requires the transaction source to be the connected account, but in SEP-10 the source is the server's signing key.
  • That is the app half of feat: wallet-authenticated sessions (challenge sign-in) for server and app #136, and I've left the details there.

[cloudflare-workers-and-pages]

Deploying puls3 with    Cloudflare Pages

Latest commit:	298648a
Status:	✅  Deploy successful!
Preview URL:	https://aae4060f.puls3-4lw.pages.dev
Branch Preview URL:	https://fix-116-freighter-bridge-una.puls3-4lw.pages.dev

View logs

[TOMOKI977]

Approved. The bridge-load failure is now distinguished from a missing Freighter installation, and the regression coverage matches that behavior.