feat(ui): render half-block thumbnails for attached images (#47)

* 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

go.mod

@@ -11,6 +11,7 @@ require (
 	github.com/alecthomas/chroma/v2 v2.26.1
 	github.com/atotto/clipboard v0.1.4
 	github.com/aymanbagabas/go-udiff v0.4.1
+	github.com/charmbracelet/colorprofile v0.4.3
 	github.com/charmbracelet/fang v1.0.0
 	github.com/charmbracelet/log v1.0.0
 	github.com/charmbracelet/openai-go v0.0.0-20260319145158-d0740cc34266
@@ -26,6 +27,7 @@ require (
 	github.com/spf13/cobra v1.10.2
 	github.com/spf13/viper v1.21.0
 	github.com/traefik/yaegi v0.16.1
+	golang.org/x/image v0.41.0
 	golang.org/x/term v0.43.0
 	gopkg.in/yaml.v3 v3.0.1
 )
@@ -55,7 +57,6 @@ require (
 	github.com/catppuccin/go v0.3.0 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/charmbracelet/anthropic-sdk-go v0.0.0-20260223140439-63879b0b8dab // indirect
-	github.com/charmbracelet/colorprofile v0.4.3 // indirect
 	github.com/charmbracelet/harmonica v0.2.0 // indirect
 	github.com/charmbracelet/lipgloss v1.1.1-0.20250404203927-76690c660834 // indirect
 	github.com/charmbracelet/x/cellbuf v0.0.15 // indirect