test(web): cover base64 encode/decode roundtrip and chunking boundary

[dirtybits]

Summary

Adds a dedicated unit test for web/lib/base64.ts — the bytes-only base64
helpers used on the auth and transaction path (wallet signatures, serialized
base64 transactions, account data). These had no dedicated test file despite
being security- and money-adjacent, while sibling boundary helpers
(chainAddress.ts, registeredAt.ts) are pinned with behavioral tests
(AGENTS.md §3.24: pure helpers get real behavioral tests, not grep-tests).

The test pins:

• encodeBase64(new Uint8Array(0)) → ""
• known-value roundtrips, including 0x00/0xff/0x80 and 12-byte input
• encodeBase64 output is byte-identical to the Buffer.from(...).toString( "base64") reference across 0, 1_000, and 10_000-byte inputs
• roundtrips across the 0x8000 String.fromCharCode chunk boundary
  (the chunkSize = 0x8000 constant exists to keep btoa spreads under the
  argument limit; sizes at and past the boundary are pinned explicitly)
• the btoa and Buffer fallback branches agree on a 2048-byte input (the
  btoa branch is hidden and restored in a finally block)

Tests / verification

• npm test --workspace @agentvouch/web -- base64 → 1 file, 5 tests passed
• Full web suite: 142 files / 1187 tests passed (no regressions)
• npm run format:check → pass
• npm run typecheck (web) → pass
• npm run lint:web → pass
• Webpack compile + TS typecheck pass. The static-generation/prerender phase
  fails in the local Linux sandbox because DATABASE_URL (a gitignored Vercel
  env secret) is absent — a pre-existing environment condition, not this change
  (the PR adds only a test file under web/__tests__, never compiled into the
  build). Vercel is the authoritative web build gate.

Not verified

• Vercel production build (gated by the platform's env); local webpack build
  is blocked on missing DATABASE_URL, attributed to the sandbox, not the diff.

Scope

One new file: web/__tests__/lib/base64.test.ts. No source changes.