[hturnbull93]

Stacked on #356.

Screenshots: https://fedora.taila91c66.ts.net:8444/docs/20261021-134117-pr-357-threaded-replies.html

Discussions on projects, tasks and quick tasks could already hold one level of replies. This PR lets people reply to any comment, quotes the specific reply answered, folds long threads, and tells the person being replied to.

API (workItemComments.add)

• A reply to a reply still lands in the same top-level thread, and the person told is the author of the comment that was replied to.
• A new replied_to_id column, distinct from parent_id, records the specific comment a reply answers. It's set only when that's a reply to a reply — a reply straight to the thread's own top-level comment already sits under it, so quoting it back would say nothing new. workItemComments.list resolves it to the target's author and a plain-text snippet (mentions stripped, cut to 140 characters), or marks it removed if the target has since been deleted.
• That person gets a new comment_reply notification: "X replied to your comment on …". It links to #comment-{id} and is sent by email unless they have muted the Updates category. The preference label already says "comments and replies".
• Nobody is notified twice. A mention outranks a reply, and a reply outranks the plain "New comment" notice that creators, owners and assignees get. The thread starter still gets "New comment" when someone replies further down their thread.

UI (CommentThreadView)

• Every live comment, replies included, has a Reply button. Its accessible name says who you are replying to ("Reply to Hal"). The reply box opens at the foot of the thread, labelled "Write a reply to Hal", and takes focus. Choosing a different comment moves the box and focus there. Submitting an empty reply still shows "Write something first."
• A reply to a reply shows "Replying to Hal: "quoted snippet"" above its own text, so it's clear which earlier reply it answers in a thread with crosstalk. A reply straight to the thread's top-level comment shows no quote — its position already says that. If the quoted comment has since been removed, it shows "Replying to a removed comment" instead.
• Each thread's replies are a nested <ul> named "Replies to ".
• A thread with more than four replies shows the first reply and the last two, with a "Show N more replies" button between them. Pressing it shows the hidden replies and moves focus to the first of them. A permalink to a hidden reply, such as the one in the notification, opens its thread.

Tests

• Router tests cover:
  • nesting depth
  • who is told, with the title, link and email
  • muted email
  • no double notification alongside a mention or the plain comment notice
  • a self-reply
  • an author whose account is gone
  • replying under a removed comment
  • an outsider refused
  • the quote on a reply to a reply, absent on a reply to the thread starter, truncated past 140 characters, and shown as removed once its target is deleted
• Component tests cover:
  • Reply labels and focus
  • answering a reply
  • the empty-reply hint
  • list markup
  • folding and expanding, with focus
  • a permalink opening a folded thread
  • the quote shown on a reply to a reply, absent on a reply to the thread starter, and worded "a removed comment" for one since deleted
• New e2e test in 07-project-management-owner.spec.ts: the owner replies, the reply nests under the comment, and the admin sees the reply notification and opens it. The test then seeds a long thread through the API, checks that it folds, and expands it. Snapshots: reply box open, long thread folded.

Decisions

• Quick tasks get threading too. They share CommentThread and the same router. Bug report threads stay flat (BugReportCommentThread doesn't pass canReply): they are a back-and-forth between the reporter and admins, on a separate model.
• Deleting and moderation: this is unchanged and already soft-deletes. A deleted comment, whether removed by its author or by a chapter admin, shows as "Comment removed" with its replies kept below it. A removed comment has no Reply button, and the API refuses a reply to it. People can still answer the remaining replies in that thread.
• Notification category: a reply counts as an update, not needs action. It shows in the Inbox's Updates and doesn't go on the dashboard's needs-action list. This matches the existing email-preference wording.
• Folding: a thread folds above 4 replies and keeps the first reply and the last two in view, so both the opening answer and the latest exchange stay visible.
• A reply to a reply doesn't prefill an @mention of the person answered. They are told through the reply notification instead.
• The quote is plain text, mentions stripped, the same treatment notification previews already get, so a long mention token can't be cut mid-token and the quote can't carry live @mention styling it has no composer to back up.
• Why a separate column instead of reusing parent_id: parent_id groups a reply under its thread's root, which the existing fold/notification logic depends on staying flat. replied_to_id is purely who-answered-whom display information layered on top, so it doesn't disturb that.

Data impact: a new nullable replied_to_id column, backfilled as empty (no way to recover which reply an existing reply answered). No change to parent_id or deleted_at.

Closes #339

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