Add Select to chat message context menus

[mcintyre94]

Summary

• Adds a Select action to the context menu on both user (blue) and assistant (grey) chat bubbles, alongside the existing Copy action
• Tapping Select enters an inline selection mode: the bubble switches to a non-editable UITextView with all text immediately pre-selected and draggable handles
• Tapping away (clearing the selection) automatically exits select mode and restores the normal bubble rendering
• Backed by a new SelectableTextView (UIViewRepresentable) that selects all text on first appear and calls back via onDeselect when the selection length drops to zero

Test plan

• Long-press a user message → context menu shows Copy and Select
• Long-press an assistant message → context menu shows Copy, Select, and (if applicable) Create Checkpoint
• Tap Select → bubble switches to text-selection mode with all text highlighted and handles visible
• Drag handles to narrow the selection → system Copy popup appears
• Tap away from the bubble → selection dismissed, bubble returns to normal
• Tap Select on assistant message → raw text shown (markdown source), selectable
• Copy still works as before on both bubble types

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