Add 'Manage Pending Invites' on friends/pending page

[ImBIOS]

Fixes/relates to: #2368

What

Adds a "Manage Pending Invites" mode to steamcommunity.com/<id>/friends/pending, mirroring Steam's native "Manage Friends List" on /friends.

The Manage Pending Invites button is placed in the Sent Invites title bar. Toggling it enters a selection mode over both received (#search_results) and sent (#search_results_sentinvites) invite rows, showing a manage panel (reusing Steam's manage_friends_panel styles) with:

• Select All / None / Inverse
• Selected count
• Bulk actions:
  • Accept (received invites) — POST /actions/AddFriendAjax with accept_invite=1, per invite (Steam has no bulk accept endpoint)
  • Ignore (received invites) — POST friends/action action=ignore_invite (same as Steam's "Ignore All")
  • Cancel Invites (sent invites, with confirm dialog) — POST friends/action action=remove (same as Steam's per-row ConfirmCancelInvite)
• Successfully handled rows are removed from the DOM and the empty-state messages update.

Why

Steam's /friends/pending has no manage mode: sent invites can only be cancelled one-by-one (each with a confirm dialog), and there's no way to bulk-accept/ignore received invites beyond the all-or-nothing "Ignore All". Users with many pending sent invites have to click through each one.

Technical notes

• The pending page's rows already carry Steam's manage-mode markup (.selectable.invite_row + .select_friend checkbox, data-steamid), and adding the .manage class shows the checkbox indicator (Steam CSS: .selectable.manage .indicator).
• Steam's own ToggleManageFriends() only styles #search_results > .selectable (received); sent invites need the .manage class applied explicitly, so this feature implements its own toggle (no page-context scriptlet needed, no g_bManage dependency).
• Own profile only (checkPrerequisites).
• New feature wired into CFriendsAndGroups (context already matches /friends/pending).

Status

Draft — UI placement, manage toggle, selection and empty-state handling verified live against the real page via a non-invasive DOM injection (no extension loaded, no data changes). The bulk AJAX calls mirror Steam's own friends.js/profile.js endpoints exactly. Live end-to-end testing of the actual POST actions (Accept/Ignore/Cancel) still pending.