feat: tray icon, chat avatars, emoji reactions, and persistent app

[fredsmith]

Summary

• Menubar tray icon: App icon moved from dock to menubar with connection state indicators (green=connected, blinking red/green=connecting, red=disconnected) and context menu (Buddy List, Go Online/Offline, Settings, Quit)
• Persistent app: Main window hides on close instead of quitting; app continues running in the tray until explicitly quit
• Chat avatars & grouping: Group chat messages show sender avatars with message grouping — successive messages from the same user display only a timestamp
• Authenticated avatar fetch: Switched from unauthenticated mxc_to_http URLs to authenticated media fetch, fixing avatars that weren't loading on some servers
• Emoji reaction picker: Full reaction picker with 32 quick-pick emoji grid plus "More..." button to open the macOS system emoji picker (Character Palette)
• Reaction history: Existing reactions now load from room history when opening a chat window
• Context menu fix: Prevent native browser context menu so custom right-click menus work reliably

Test plan

• Verify tray icon appears in menubar and reflects connection state (connecting/connected/disconnected)
• Verify tray context menu works: Buddy List opens window, Go Online/Offline toggles, Settings opens preferences, Quit exits app
• Verify closing the main window hides it (app stays in tray)
• Verify chat room messages show avatars and group successive messages from same sender
• Verify right-click context menus work on messages and participant list
• Verify "React..." opens emoji picker with color emoji
• Verify "More..." button opens macOS system emoji picker
• Verify existing reactions display when opening a chat window
• Verify new reactions sent via the picker appear in other clients

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