Threaded (nested) comments with a collapsible, connector-line UI

[ItsMalikJones]

Closes #11

Summary

Adds opt-in threaded comment replies to Commentions. Users can reply directly
to a comment, replies render recursively beneath their parent, and long
sub-threads can be collapsed. The feature is disabled by default and gated behind
a new config block, so existing installations are unaffected until they opt in.

What's included

Data layer

• Additive migration — a new nullable, self-referencing parent_id column on
  the comments table (add_parent_id_to_commentions_comments_table.php.stub),
  with a foreign key that cascades on delete.
• Comment model — parent() / replies() relations, a depth() helper
  that walks the parent chain, and repliesCount() which recursively counts all
  descendants.
• HasComments::commentsQuery() — returns only top-level comments when
  threading is enabled, and eager-loads replies down to the configured
  max_depth to avoid N+1 queries.
• SaveComment — accepts an optional parentId so replies persist with the
  correct parent.
• Deleting a comment recursively deletes its replies.

UI

• A Reply control on each comment opens an inline editor; saveReply()
  persists the reply via SaveComment. The control is hidden once a comment
  reaches max_depth, both in the UI and enforced server-side.
• Replies render recursively, each indented beneath its parent.
• Collapsible sub-threads — every comment with replies gets an accessible
  toggle (<button> with aria-expanded / aria-controls) that collapses or
  expands its sub-thread and shows a descendant count ("3 replies") when
  collapsed. Collapsing is handled client-side via Alpine, so no extra server
  round-trips and nested component state is preserved.
• Facebook-style connector lines — replies are joined to their parent by a
  curved elbow connector (a continuous vertical trunk with a rounded branch into
  each reply's avatar). The trunk ends cleanly at the last reply. Drawn with pure
  CSS, including dark-mode colours.
• Capped indentation — horizontal indent stops being added past depth 2 so
  deep threads stay readable on small screens; the connector line still renders
  at every level.
• New English strings (replies_count, hide_replies) plus a reply key added
  across all bundled locales (ar, en, es, fr, nl, ro).

Configuration

A new commentions.threading config block:

'threading' => [
    'enabled'   => env('COMMENTIONS_THREADING_ENABLED', false),
    'max_depth' => (int) env('COMMENTIONS_THREADING_MAX_DEPTH', 3),
],

Threading is disabled by default. max_depth is the deepest reply level
allowed — top-level comments are depth 0, so the default of 3 permits replies
down to depth 3.

Backward compatibility

• Non-breaking. With threading.enabled = false (the default), behaviour is
  identical to today — a flat comment list.
• The migration is purely additive (new nullable column); existing comments
  become top-level comments (parent_id = null).
• No changes to existing public method signatures (SaveComment's new argument
  is optional).

Testing

• 16 feature tests in tests/Livewire/CommentThreadingTest.php cover: creating
  replies, depth calculation, top-level-only querying, reply-form behaviour,
  max_depth enforcement (UI + server), cascade deletion, repliesCount(),
  card-vs-flat rendering by depth, the collapse toggle markup and ARIA
  attributes, the reply-count label, connector rendering, and the indentation
  cap.