feat: add Followers page to user profiles

Closes #46 (2.x port of #49).

**New features**
- New `/u/:username/followers` route showing who follows a given user
- "Followers" nav link appears on any user's profile (public information)
- "Following" nav link remains restricted to the logged-in user's own profile

**Bug fixes**
- `FollowingPage` (formerly `ProfilePage`) now correctly loads the user
  via `loadUser(m.route.param('username'))` + `show()` override, instead
  of hardcoding `app.session.user` — the page now works when visited
  directly by URL for any user
- Added null guard on `this.user` in `navItems` extend

**Performance**
- Removed `followedBy` from `Index` endpoint default includes — was
  loading the full followers list for every user in search/list results

**Refactoring**
- `ProfilePage` renamed to `FollowingPage` for clarity
- `FollowedUserListItem` renamed to `UserListItem` (now shared between
  both FollowingPage and FollowersPage)

**Locale**
- Added `followers_link` key to `forum` top-level (consistent with
  existing `profile_link`)
- Added `profile_page.no_followers` placeholder string

**Tests**
- Added `FollowedByTest` with 3 integration tests:
  - `followedBy` relation is included when viewing another user's profile
  - `followedBy` relation is included when viewing own profile
  - `followedBy` is NOT included in the forum boot response (regression
    guard for the perf fix)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: imorland

extend.php

@@ -60,7 +60,7 @@
     (new Extend\ApiResource(Resource\UserResource::class))
         ->fields(Api\UserResourceFields::class)
         ->endpoint(Endpoint\Index::class, function (Endpoint\Index $endpoint) {
-            return $endpoint->addDefaultInclude(['followedUsers', 'followedBy']);
+            return $endpoint->addDefaultInclude(['followedUsers']);
         })
         ->endpoint(Endpoint\Show::class, function (Endpoint\Show $endpoint) {
             return $endpoint->addDefaultInclude(['followedUsers', 'followedBy']);