docs(wallet): document multi-wallet flow, remove dead FreighterAdapter

[jerrymusaga]

Summary

Issue #74 asked for xBull wallet support on desktop. Investigation found that xBull (along with Lobstr, Hana, Rabet) is already fully supported end-to-end via Stellar Wallets Kit (src/lib/stellarWalletsKit.ts), which has been the connect/sign backbone since an earlier migration off the direct Freighter/Albedo integration referenced in the issue's original file list. There was no missing per-provider connection or signing logic to add.

What this PR does instead:

• Removes src/wallet/FreighterAdapter.ts and its re-export in src/wallet/index.ts — dead code kept "for back-compat" with no real consumers; the app uses StellarWalletAdapter, which delegates everything to the kit.
• Adds a "Wallet providers" section to docs/architecture.md documenting the current provider-agnostic connect/sign flow and xBull's desktop behavior specifically.

Before / after

Before: src/wallet/ still exposed a single-provider FreighterAdapter (unused) alongside the real StellarWalletAdapter; docs/architecture.md described auth as "Freighter/Albedo" only, with no mention of xBull, Lobstr, Hana, or Rabet, and no explanation of how a new provider is added.

After: src/wallet/ only exposes the adapter that's actually used. docs/architecture.md documents that connect/sign go through Stellar Wallets Kit's own modal, lists all six registered wallet modules, and explains that adding a new provider only requires registering its module in stellarWalletsKit.ts. Existing desktop (Freighter, xBull, Lobstr, Hana, Rabet) and mobile (Albedo) behavior is unchanged — this is a doc + dead-code cleanup, not a behavior change.

What's supported / not supported (per issue's acceptance criteria)

• xBull can connect and sign — via the kit's shared modal, same as every other desktop provider. No xBull-specific code exists or is needed.
• Existing desktop wallet behavior (Freighter, Lobstr, Hana, Rabet) is untouched.
• Not covered by this PR: any new wallet integration — none was needed, since xBull was already registered in stellarWalletsKit.ts (xBullModule) prior to this change.

Test plan

• tsc --noEmit passes clean after removing FreighterAdapter.ts
• Confirmed no remaining references to FreighterAdapter in src/
• Manual: connect + sign a transaction with xBull on desktop (reviewer/maintainer to confirm in their environment, since this PR makes no functional change to that path)

Closes #74

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