refactor(tui): extract mode-specific state into sub-models

[piekstra]

[#14]

Extract mode-specific state from the flat Model struct into dedicated sub-types for better organization and clarity.

Changes

New sub-model types:

• MainState: Holds state for the main two-pane view (pendingOps, showConfirm, showQuitConfirm, mouseEnabled)
• FilterState: Holds state for filter mode (active, text)
• ProgressState: Holds state for operation progress (operations, errors, currentIdx, loading)

Updated Model struct:

type Model struct {
    // Core state (always used)
    client      claude.Client
    styles      Styles
    keys        KeyBindings
    plugins     []PluginState
    filteredIdx []int
    mode        Mode
    height      int
    width       int
    selectedIdx int
    listOffset  int
    err         error
    workingDir  string

    // Mode-specific state (value types)
    main     MainState
    filter   FilterState
    progress ProgressState
}

Benefits

1. Clearer organization - Mode responsibilities are explicit
2. Easier testing - Can test mode state in isolation
3. Better documentation - Type names document purpose
4. Future-proof - Adding new modes doesn't pollute main struct
5. Initialization clarity - Each mode can have its own constructor

Migration

All existing functionality preserved. Field accesses updated to use sub-models:

• m.pendingOps → m.main.pendingOps
• m.filterActive → m.filter.active
• m.operations → m.progress.operations
• etc.

Closes #14