refactor(tui): SessionList and PreviewPanel components are god objects with 54 and 34 methods

[jongio]

Problem

SessionList in internal/tui/components/sessionlist.go has 54 methods and PreviewPanel in internal/tui/components/preview.go has 34 methods. Both exceed a reasonable threshold for single-responsibility structs and qualify as god objects.

SessionList (54 methods) mixes concerns

Concern	Example Methods
Cursor/scroll	MoveUp, MoveDown, MoveTo, ScrollOffset, ScrollBy
Selection	Selected, SelectAll, DeselectAll, SelectByID, SelectRange, ToggleSelected
State management	SetSessions, SetGroups, SetPivotField, SetSize
Tree mode	CollapseAll, ExpandAll, ToggleFolder, CollapseFolder, ExpandFolder
Filters	SetHiddenSessions, SetFavoritedSessions, SetAISessions, SetAttentionStatuses, SetWorkStatuses, SetPlanStatuses
Rendering	View, renderSessionRow, renderFolderRow, attentionDot, planDot, workStatusDot

PreviewPanel (34 methods) mixes concerns

Concern	Example Methods
Detail display	SetDetail, Content, HasPlanContent, PlanViewMode
Scrolling	ScrollUp, ScrollDown, PageUp, PageDown, ScrollOffset
Plan mode	SetPlanContent, ShowPlanView, ExitPlanView, TogglePlanView
Text selection	StartSelection, UpdateSelection, FinalizeSelection, SelectedText, HasSelection
Rendering	View, renderContent, renderPlanContent, updateTotalLines
Status	SetAttentionStatus, SetWorkStatus, SetConversationSort

Suggested approach

Extract cohesive method groups into focused helper types:

• SessionList: extract a SessionListRenderer (rendering methods) and SelectionManager (selection logic), keep SessionList as the coordinator.
• PreviewPanel: extract a PreviewRenderer (View, renderContent, renderPlanContent) and TextSelectionHandler (StartSelection, UpdateSelection, FinalizeSelection, SelectedText).

This makes each type easier to test and reason about in isolation.

[jongio]

Consolidated into a single tracking issue.