fix(web): complete accessible form labeling and error associations

[jeanduplessis]

Problem

Forms across authentication, organization management, automation, App Builder, deployments, Gastown, Wasteland, BYOK, and admin rely on placeholders, unassociated labels, or visual-only validation messages.

Remediation action

Give every audited field a persistent accessible identity and programmatically associated validation state.

Representative scope

• apps/web/src/components/auth/sign-in/EmailInputForm.tsx:34
• apps/web/src/components/profile/EditProfileDialog.tsx:126
• apps/web/src/components/organizations/members/InviteMemberDialog.tsx:217
• apps/web/src/components/organizations/byok/BYOKKeysManager.tsx:661
• apps/web/src/components/webhook-triggers/TriggerForm.tsx:581
• apps/web/src/components/app-builder/PromptInput.tsx:338
• apps/web/src/components/deployments/PasswordFormFields.tsx:106
• apps/web/src/components/gastown/CreateTownDialog.tsx:58
• apps/web/src/app/(app)/wasteland/new/NewWastelandWizardClient.tsx:665
• apps/web/src/app/admin/alerting-ttfb/TtfbAlertingContent.tsx:195
• apps/web/src/app/admin/gateway/RoutingContent.tsx:91

Acceptance criteria

• Every field has a visible Label where practical, with matching htmlFor and id; placeholder text is supplementary only.
• Invalid controls expose aria-invalid and reference stable help/error IDs through aria-describedby.
• Mutation-level errors use appropriate live-region semantics and provide recovery guidance.
• Password visibility and similar field controls remain keyboard reachable and state-labeled.
• Validation occurs on blur or submit unless immediate validation is intentionally required.
• Tests cover label association and announced validation for representative forms.