[Client] Tabbed room channels with unread states and collapsible chat (#7478)

* [Client] Add room channel selector with per-channel chat views

* [Client] Tabbed room channels with unread states, collapse, read-only, banners

Reworks the room chat channel display: channels move out of the combobox
into a tab bar below the message input showing every channel available
to the user.

- Each tab shows its unread message count and is tinted when it holds
  unreads or a mention; both colors are user-configurable chat settings
  (channelUnreadColor, channelMentionColor) with a settings-page section.
- Clicking the active tab collapses the chat: message view and input
  hide and the splitter section minimizes to the tab strip; expanding
  restores the previous splitter sizes. Viewing a channel clears its
  unread state.
- Channels the user may read but not write (per write_access_level)
  show with the message input disabled and a read-only placeholder;
  sends are guarded locally, the server still enforces.
- Channel banner_message strings render at the top of their channel
  view on join, like the room join message on Main.
- Replayed channel history no longer inflates unread counts; moderation
  redaction now applies across all channel views.

* Fixup: keep chat tab bar visible when chat is collapsed

QSplitter children are collapsible by default, so setSizes({1, 0})
hid the whole chat group box including the channel tab bar.
Mark the chat widget non-collapsible so collapsing clamps to its
minimum size (title + tab bar) instead.

* [Room] Explicitly initialize MAIN_CHANNEL per review

* [Room] Address channel tab review follow-ups

- gate @all mentions on moderator/admin sender to match ChatView,
  whole-word matching for own name and highlight words
- hide single-channel tab bar so old servers do not show a lone Main
  tab that collapses chat on misclick
- capture target ChatView in sendMessage so errors land in the
  channel the message was sent from
- share ignore/unregistered filtering between RoomSay and
  RoomChannelSay via isRoomMessageIgnored

* [Client] Fix tab_room.cpp formatting per clang-format-18

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>
Co-authored-by: BruebachL <bruebachl@cockatrice.example>

libcockatrice_interfaces/libcockatrice/interfaces/interface_chat_settings_provider.h

@@ -12,6 +12,8 @@ public:
     [[nodiscard]] virtual bool getChatMentionCompleter() const = 0;
     [[nodiscard]] virtual QString getChatMentionColor() const = 0;
     [[nodiscard]] virtual QString getChatHighlightColor() const = 0;
+    [[nodiscard]] virtual QString getChannelUnreadColor() const = 0;
+    [[nodiscard]] virtual QString getChannelMentionColor() const = 0;
     [[nodiscard]] virtual bool getChatMentionForeground() const = 0;
     [[nodiscard]] virtual bool getChatHighlightForeground() const = 0;
     [[nodiscard]] virtual bool getIgnoreUnregisteredUsers() const = 0;