feat: add CLI with TUI, self-update, install script, and terminal section on landing page

- Full-screen Bubble Tea TUI with cream background fill using PadLine/FillBlankLines
- Self-update command (--update) pulling from GitHub releases
- install.sh for curl one-liner installation
- Terminal Lovers section on web landing page with install command and CLI features
- All 7 format categories, glob/directory batch support, auto-download ffmpeg

cli/internal/tui/keys.go

@@ -0,0 +1,78 @@
+package tui
+
+import "github.com/charmbracelet/bubbles/key"
+
+// KeyMap defines key bindings for the TUI.
+type KeyMap struct {
+	Up        key.Binding
+	Down      key.Binding
+	Left      key.Binding
+	Right     key.Binding
+	Enter     key.Binding
+	Space     key.Binding
+	Tab       key.Binding
+	Delete    key.Binding
+	SelectAll key.Binding
+	Convert   key.Binding
+	Quit      key.Binding
+	Help      key.Binding
+	Back      key.Binding
+}
+
+// DefaultKeyMap returns the default key bindings.
+func DefaultKeyMap() KeyMap {
+	return KeyMap{
+		Up: key.NewBinding(
+			key.WithKeys("up", "k"),
+			key.WithHelp("↑/k", "up"),
+		),
+		Down: key.NewBinding(
+			key.WithKeys("down", "j"),
+			key.WithHelp("↓/j", "down"),
+		),
+		Left: key.NewBinding(
+			key.WithKeys("left", "h"),
+			key.WithHelp("←/h", "prev format"),
+		),
+		Right: key.NewBinding(
+			key.WithKeys("right", "l"),
+			key.WithHelp("→/l", "next format"),
+		),
+		Enter: key.NewBinding(
+			key.WithKeys("enter"),
+			key.WithHelp("enter", "confirm"),
+		),
+		Space: key.NewBinding(
+			key.WithKeys(" "),
+			key.WithHelp("space", "toggle select"),
+		),
+		Tab: key.NewBinding(
+			key.WithKeys("tab"),
+			key.WithHelp("tab", "cycle format"),
+		),
+		Delete: key.NewBinding(
+			key.WithKeys("d", "delete", "backspace"),
+			key.WithHelp("d", "remove file"),
+		),
+		SelectAll: key.NewBinding(
+			key.WithKeys("a"),
+			key.WithHelp("a", "select all"),
+		),
+		Convert: key.NewBinding(
+			key.WithKeys("c"),
+			key.WithHelp("c", "convert"),
+		),
+		Quit: key.NewBinding(
+			key.WithKeys("q", "ctrl+c"),
+			key.WithHelp("q", "quit"),
+		),
+		Help: key.NewBinding(
+			key.WithKeys("?"),
+			key.WithHelp("?", "help"),
+		),
+		Back: key.NewBinding(
+			key.WithKeys("esc"),
+			key.WithHelp("esc", "back"),
+		),
+	}
+}