fix: return viewer invitation validation feedback

[richiemcilroy]

Invalid viewer email input currently throws from the invitation Server Action. Return the expected validation failure after owner authorization, then show “Enter a valid email address” without clearing the input, refetching grants, refreshing the page, or showing success feedback. Disable editing while a submission is pending so completion cannot erase a newer address.

The existing email pattern, 254-character limit, trimming/lowercasing, owner checks, valid invitation flow, and revoke behavior are preserved. Unexpected grant failures still throw. No schema or environment changes.

Related: CAP-WEB-3H. The submitted production address and actual feedback are unavailable; this change does not assume valid invitations were broken. Existing overlapping organization-access PRs do not handle this validation result.

Validation:

• Passed 98 tests across five suites: invitation action, sharing dialog, sharing defaults, viewer access policy, and audience descriptions.
• Regression coverage includes invalid input with no downstream effects, authorization before validation, normalized valid input and the length boundary, duplicate/revoked invitations, delivery failures, repeated submissions, and correction after validation feedback.
• Passed scoped Biome, web route type generation, web/dependency TypeScript checks, and whitespace checks.
• Used isolated synthetic fixtures with mocked database/email boundaries. Live invitation delivery and a production browser flow were not run.