feat: implement WebSocket connection and channel subscription functionality

[quytrandinh-ncc-asia]

Summary

Replace hardcoded test channel subscription with a store-driven flow where the transport task polls the ChannelList for pending subscription changes and executes leave/join transitions accordingly.

Changes

mezon-store

• Added PendingSubscribe struct (clan_id, channel_id, channel_type, is_public)
• Added pending_subscribe: Option field to ChannelList
• Added ChannelType::as_proto_int() helper to convert enum to i32 for protobuf

mezon-client

• Updated subscribe_channel signature to accept channel_type: i32, is_public: bool (replacing hardcoded 0, true)
• Added fire-and-forget leave_channel method using direct adapter.send() without response tracking
• Added TransportClient delegates for both methods

mezon-ui

• ChannelSidebar.on_channel_click now sets ChannelList.pending_subscribe with real channel data (looks up Channel to get channel_type and is_public)
• ChatLayout accepts Entity from constructor instead of creating internally
• RootView accepts and threads Entity through to ChatLayout

mezon-app

• Created ChannelList entity in run_app() before opening main window
• Passed Entity to both open_main_window (→ UI tree) and spawn_transport_task
• Transport task polls pending_subscribe every 500ms: if changed, sends ChannelLeave for old channel (fire-and-forget) then ChannelJoin for new
• Removed hardcoded subscribe_channel call after get_account
• On reconnect, resets subscription tracking to force re-subscribe on next poll cycle

Behavior

• User clicks a channel → pending_subscribe set
• Transport task (500ms poll) detects change → leaves old channel (if any) → joins new channel
• Channel switches → leave old/join new happens automatically
• WebSocket reconnects → resubscribes to currently selected channel
• Uses real channel metadata (channel_type, is_public) instead of hardcoded values

All tests pass: cargo check -p mezon-app succeeds.