feat: commit info overlay with m key (#103)

## Summary
- Add `m` key to open a floating overlay showing the full commit
preamble (hash, author, date, message body)
- Display commit metadata in the header bar: `DIFFNAV • hash date author
• [branch] subject`
- Clicking the header bar opens the overlay; clicking outside closes it
- Scrollable viewport with scrollbar when content overflows (mouse
wheel, j/k, up/down, ctrl+d/ctrl+u for half-page)
- Branch detection via `git branch --points-at` for any commit
- Does nothing for plain diffs without commit metadata

<img width="1831" height="1073" alt="image"
src="https://github.com/user-attachments/assets/03cdc2bb-58ad-4105-be3a-18d1694cca31"
/>

Author: pablospe

pkg/ui/keys.go

@@ -21,6 +21,7 @@ type KeyMap struct {
 	ToggleDiffView  key.Binding
 	ToggleIconStyle key.Binding
 	ToggleHelp      key.Binding
+	ToggleMessage   key.Binding
 }
 
 var keys = &KeyMap{
@@ -96,6 +97,10 @@ var keys = &KeyMap{
 		key.WithKeys("?", "f1"),
 		key.WithHelp("F1/?", "toggle help"),
 	),
+	ToggleMessage: key.NewBinding(
+		key.WithKeys("m"),
+		key.WithHelp("m", "commit info"),
+	),
 }
 
 func KeyGroups() [][]key.Binding {
@@ -115,6 +120,7 @@ func KeyGroups() [][]key.Binding {
 		keys.ToggleDiffView,
 		keys.ToggleIconStyle,
 	}, {
+		keys.ToggleMessage,
 		keys.ToggleHelp,
 		keys.Quit,
 	}}