* feat(ui): render half-block thumbnails for attached images (#46)
- Add internal/ui/imagepreview package: Render() draws low-res
thumbnails using Unicode half-blocks (▀) + truecolor/256-color SGR,
which survives tmux/zellij (no graphics protocol)
- Cache a rendered thumbnail per pending clipboard image in the input
component; render once at attach time, never per frame
- Fall back to the existing [N image(s) attached] text pill when the
terminal lacks truecolor/256-color support
- Document Ctrl+V paste, Ctrl+U clear, and the preview in the docs
site and README keyboard shortcuts
Fixes#46
* fix(ui): render image thumbnails off the event loop and cap size
- Render thumbnails asynchronously via a tea.Cmd instead of calling
the decode + resample path synchronously inside Update(), which
blocked the Bubble Tea event loop
- Add thumbnailReadyMsg + an imageGen generation counter so async
results land on the correct pendingImages slot and stale renders
after a clear/re-attach are discarded
- Guard imagepreview.Render against decompression bombs by checking
DecodeConfig dimensions against a max before full decode
* fix(ui): skip image preview when input width is too small
- Return 0 from thumbCols when width <= 6 so a full-size thumbnail is
no longer rendered for tiny or uninitialized (width 0) terminals;
the caller falls back to the text pill
Ed Zynda