AAC-Native UI: Full UI Architecture Skeleton

[trancer1994]

This PR introduces the full AAC-native replacement UI architecture for the TeamTalk Qt client. It defines all major screens, navigation flow, and state-driven transitions without yet implementing networking, audio, or channel/user data. The goal is to establish a clear, maintainable UI structure that future PRs can progressively wire up to real functionality.

Included

• New main screens:
  • Server list view (existing: ServerListWidget)
  • Public servers placeholder view (PublicServerListWidget)
  • Connection view (ConnectionView)
  • Channel list view (ChannelView)
  • In-channel view (InChannelView)
  • Settings dialog (SettingsDialog)
• Basic models:
  • ServerEntry
  • ServerListModel
• State integration:
  • UiState enum and basic transitions for:
    • Server list → Connecting → Channel list → In-channel → back again
  • Wiring between UI widgets and the existing state machine introduced earlier
• Navigation:
  • Central widget swapping in MainWindow based on UI state
• Placeholders:
  • Text chat placeholder in InChannelView
  • Placeholder tabs in SettingsDialog
  • Connection status text in ConnectionView
  • Public server list placeholder in PublicServerListWidget

Not included

• Any actual networking or TeamTalk protocol calls
• Real channel list population
• Real user list population
• Audio device handling
• Voice transmit/mute logic
• Server discovery beyond what TeamTalk already exposes
• Full AAC styling (button size, spacing, theme, scanning order); these will follow in later PRs once the structure is agreed.

Purpose
The purpose of this PR is to expose the complete AAC-native UI structure as a coherent whole, so reviewers can see:

• All major screens
• The intended navigation flow
• How the replacement UI fits into the existing client
• How the state machine drives UI transitions

Future PRs will:

• Connect the state machine to the actual TeamTalk backend
• Populate channels and users
• Implement transmit/mute behavior
• Add AAC-native sizing, spacing, and input models (switch/scanning/eye-tracking support)
• Refine accessibility and theming.

Notes for reviewers

• All new screens are intentionally minimal and UI-only.
• The focus here is on layout, separation of responsibilities, and predictable navigation.
  Note to developers: anytime I refer to "functionality" in my commits, it is either minimal or placeholder.
  I will edit this request if I find any files are missing, or anything needs improving, or if I otherwise get feedback.