Add the Follow toggle and the document-change notification to the app #205

Problem

A person cannot see or change document-follow settings in the app. There is no email category for document changes, no per-document Follow control, and no in-app entry for the new notification.

A content_change notification carries no sender. NotificationItem treats any sender-less notification as a system alert (apps/webapp/src/components/notificationPanel/components/NotificationItem.tsx:17-18). Without a new branch, that row renders the warning triangle (:78-81) and the name "System" (:93). Its button reads "Review" in the warning variant (:137, :142). Clicking it only toasts "Check your email settings in Profile → Notifications" (:35-40).

Part of #169.

What to do

Backend lands first. This issue adds no SQL.

• content_change is not one of the eight notification_category enum values (apps/webapp/src/types/supabase.ts:1660-1668).
• set_document_follow and get_document_follow_state do not exist yet. git grep returns no hit for either name.
• The enum value and both RPCs come from the backend issues in this set.

Global category toggle.

• Add email_content_changes?: boolean to NotificationPreferences (apps/webapp/src/components/settings/types.ts:36), beside email_reactions (:47).
• Default it to true in the state init of NotificationsSection (apps/webapp/src/components/settings/components/NotificationsSection.tsx:317-321), beside email_reactions: false (:320).
• Add one ToggleRow (:224) after the Reactions email row (:597-604), inside the preferences.email_enabled block (:579). Label "Document changes". Description "When a document you follow is edited".
• Wire it through the existing debounced handlePreferenceChange (:362). No SQL change: update_notification_preferences merges the patch with || and accepts a new key (packages/supabase/scripts/07-0-notifications.sql:41-68).

RPC wrappers. Add getDocumentFollowState.ts and setDocumentFollow.ts under apps/webapp/src/api/rpc/, beside joinWorkspace.ts, and export both from apps/webapp/src/api/rpc/index.ts. Await each call.

Per-document Follow toggle.

• Add one ToggleSection (apps/webapp/src/components/TipTap/toolbar/desktop/DocumentSettingsPanel.tsx:20) into the soft well (:82).
• Render it for any signed-in user, so it must sit outside the owner-only branch. The owner branch is :104-126. The non-owner branch is :127-132.
• The toggle description reads "Notify me when this document changes." The mute is applied in the fan-out recipient query, upstream of the notification insert. So unfollowing removes the in-app row as well as the email. "Email me…" would promise less than the switch does.
• Disable it until joinedWorkspace is true (apps/webapp/src/stores/workspace.ts:35), because set_document_follow is update-only and needs the membership row.
• Flip optimistically and revert on error. Follow the design-system skill for the visuals.

Widen the notification type. TNotification (apps/webapp/src/types/domain.ts:53-69) has no action_url, and both channel_id (:65) and message_id (:67) are non-nullable. Add action_url?: string | null, and let the two ids be nullable. Both fetch RPCs already return the column (packages/supabase/scripts/10-functions.sql:669, packages/supabase/scripts/07-1-notification-functions.sql:50).

In-app entry.

• NotificationIcon: add a content_change entry to iconMap (apps/webapp/src/components/notificationPanel/components/NotificationIcon.tsx:7-17), using Icons.pencil (apps/webapp/src/components/icons/registry.ts:160) and the label "Document change".
• NotificationItem: add the content_change click branch in handleViewNotification before the isSystemNotification branch (:34-40). Mark the row read, dismiss the panel, and navigate to action_url when it differs from the current path.
• isSystem (:73) drives five render sites. Take all five into account:
  • the warning avatar (:78-81)
  • the "System" name (:93)
  • the copy-link button gate (:103-113)
  • the button variant (:137)
  • the button text (:142)
• A sender-less content_change must show the document-change icon, its document name, and a plain "View" button. It must not show the warning avatar or "Review".
• Keep the copy-link button hidden for content_change. Today !isSystem (:103) already hides it. It stops hiding it once the new branch makes the row non-system. handleCopyUrl (:57) writes msg_id and chatroom from the row (:59-60). A carrier has no message, so the copied link is broken.

Acceptance

• "Document changes" appears in Profile → Notifications under email, only while email notifications are on, and its state survives a page reload.
• A signed-in non-owner sees the Follow toggle in the document settings panel. An anonymous visitor sees no Follow control.
• The Follow toggle is disabled until the workspace join finishes, then it enables.
• Turning Follow off, reloading, and reading the state back returns the same value. The same holds turning it on.
• Turning Follow off leaves the member roster's last_visit_at for that user unchanged. That column falls back to workspace_members.updated_at (packages/supabase/scripts/10-8-func-workspace_members.sql:238), so a careless write shows as a fresh "Last seen".
• The Follow description reads "Notify me when this document changes."
• A second user edits the document. The first user's bell shows one entry with the document-change icon, no warning triangle, no "System" name, and a "View" button. Clicking it navigates to the document and clears the badge.
• The copy-link button is absent on a content_change entry.
• Chat notification rows and the chat digest look unchanged.
• bun run --filter @docs.plus/webapp typecheck is clean.
• Verified in a browser in light and dark, on desktop and on mobile, against a running dev server. Never run next build against a live dev server.

Notes

No new unit or Cypress tests. The full flow is pinned by the backend end-to-end test in the sibling issues, and a mocked duplicate here adds nothing.