feat: native text selection with keyboard and mouse

[oobagi]

Problem

There is no text selection support in the app. Users cannot select text with Shift+arrow keys or mouse click-drag, and cannot copy selected text to the clipboard. This makes it impossible to copy content from notes without relying on terminal-native selection, which doesn't respect the app's viewport or line wrapping.

Context

The current codebase has zero selection infrastructure:

• No selection state model in the textarea (fork/bubbles/textarea/textarea.go)
• No click-drag mouse handlers
• No selection highlight rendering
• No clipboard integration beyond the existing "copy path" command in browser

Mouse interaction is currently limited to:

• View mode: Hover tracking and checklist toggle (editor.go:803-852)
• Edit mode: No mouse interaction at all

The viewport (fork/bubbles/viewport/viewport.go) handles mouse wheel events but has no concept of selection ranges.

Possible approaches

Approach A: Phased implementation in the forked textarea

• How: Build selection directly into fork/bubbles/textarea/textarea.go:
  1. Add selection anchor/cursor range tracking
  2. Implement Shift+arrow key selection
  3. Add mouse click-drag selection via MouseMsg events
  4. Render selection highlight in the textarea's View()
  5. Clipboard integration (Ctrl+C / Cmd+C to copy)
• Pros: Full control, deeply integrated, works across wrapped lines
• Cons: Significant effort, needs careful coordinate math across wrapped lines
• Files: fork/bubbles/textarea/textarea.go, internal/editor/editor.go

Approach B: View-mode only selection via viewport

• How: Implement selection only in view mode, where the viewport renders static content. Track selection as absolute line:col ranges in the viewport.
• Pros: Simpler — no interaction with textarea editing. Covers the primary "read and copy" use case.
• Cons: No selection in edit mode. Two different selection systems if edit mode is added later.
• Files: fork/bubbles/viewport/viewport.go, internal/editor/editor.go

Approach C: Defer to terminal-native selection

• How: Ensure the app doesn't interfere with terminal-native selection (some TUI mouse capture disables it). Add a modifier key (e.g., Option+click) to bypass mouse capture.
• Pros: Zero implementation effort, leverages existing terminal behavior
• Cons: Doesn't respect app layout, breaks on wrapped lines, not "native-feeling"
• Files: internal/editor/editor.go (mouse capture toggle)

Tasks

• Design selection state model (anchor position, cursor position, direction)
• Implement Shift+arrow key selection in textarea (character, word, line granularity)
• Implement mouse click-drag selection with MouseMsg events
• Render selection highlight (background color) in textarea View()
• Implement selection in view mode viewport
• Add Ctrl+C / Cmd+C to copy selection to system clipboard
• Handle selection across block boundaries in editor
• Clear selection on non-selection actions (typing, cursor move without Shift)
• Test selection across soft-wrapped lines

Test plan

• Shift+Right/Left selects characters, highlight visible
• Shift+Up/Down selects lines
• Shift+Cmd+Right/Left selects to line start/end
• Mouse click-drag creates selection in edit mode
• Mouse click-drag creates selection in view mode
• Ctrl+C / Cmd+C copies selected text to system clipboard
• Selection renders correctly across soft-wrapped lines
• Selection clears when typing or moving cursor without Shift
• Double-click selects word, triple-click selects line (if implemented)
• No regression: existing keyboard navigation still works

Scope

Type: feature
Size: large — consider phased implementation