feat: SAS code entry replaces visual comparison

[YairEtzion]

Summary

• Controller displays the 6-digit SAS code, no longer prompts for confirmation
• Target prompts the operator to type the code shown on the controller's screen
• Constant-time verifySAS() comparison catches mismatches automatically — eliminates rubber-stamp risk from "Codes match? (Y/n)" defaulting to Y
• One-sided verification on the target is sufficient: the target's allow list is the security-critical one (controls who may authenticate)

Changes

• packages/cli/src/handshake.ts — added verifySAS() with constant-time comparison
• packages/cli/src/commands/listen.ts — replaced confirm prompt with code entry prompt
• packages/cli/src/commands/invite.ts — display-only (removed confirm prompt and unused imports)
• packages/relay/src/__tests__/handshake.integration.test.ts — 5 new tests for verifySAS
• Updated docs: protocol-spec, architecture-decisions, guide, self-hosting, CLI README, root README
• Updated landing page: home page demo, self-hosting docs page

Test plan

• All existing tests pass (bun run test — 157 tests across 4 packages)
• 5 new verifySAS tests: matching codes, mismatched codes, wrong length, empty input, leading zeros
• Lint passes (bun run lint)
• Build passes (bun run build)

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