ui: dynamic A-F buttons in keypad that only appear in HEX mode

- A-F (hex digits) now render as additional top row in the keypad *only* when CurrentBase() == HEX (i.e., after Tab).
- Used dedicated hexBtn + hexKey style (orange accent) for the A-F row to avoid label conflict with the 'C' clear button.
- Pressed flash works for A-F via existing pressedKey mechanism (set in the HEX entry keyboard handler).
- The numeric 4-row grid remains unchanged in other modes.
- Updated UI_DESIGN.md layout sketch and description to document the dynamic behavior.
- Matches the galculator-inspired 'only when in hex mode' and keeps the keypad clean otherwise.
- Build verified.

docs/UI_DESIGN.md

@@ -33,24 +33,33 @@ The keypad is decorated to evoke a physical, tactile calculator using only the k
   - Operators (+ - * / =): accent color 63 for quick recognition.
   - Clears (C, AC): red-tinted bg 52 + fg 203 (warning/danger).
   - MOD: highlighted (214 orange).
-  - BASE: prominent border color 63 + bold (stands out as the special display function).
+  - (A-F appear only in HEX mode — see below)
 - Pressed state: the key inverts to the bright flashStyle (white fg on 63 bg, bold) for ~140ms on every action. This provides immediate "tactile click" visual feedback (modeled directly on gostations volume/skip/stop flashes).
 - Keys use Height(2) for chunkier, more button-like presence.
 - Spacing ("  " gaps) between keys.
 
 The keys sit directly on the main card background (no faceplate container).
 
-Layout sketch (subject to refinement; 4-5 columns):
+Layout sketch (subject to refinement; dynamic based on mode):
 
+Normal mode (DEC/BIN/OCT):
 ```
   7  8  9  /  MOD
   4  5  6  *  C
   1  2  3  -  AC
   0  . +/-  +  =
-         BASE
 ```
 
-The BASE action (cycle display format) is available via the `Tab` key (documented in the hint row). There is no on-screen BASE button in the keypad grid. This keeps the button layout regular and avoids awkward single-button rows, while still making the feature easily discoverable via the persistent hint.
+HEX mode (A-F row appears only after Tab into HEX):
+```
+  A  B  C  D  E  F
+  7  8  9  /  MOD
+  4  5  6  *  C
+  1  2  3  -  AC
+  0  . +/-  +  =
+```
+
+The BASE action (cycle display format) is available via the `Tab` key (documented in the hint row). There is no on-screen BASE button. A-F buttons appear dynamically only when the base indicator shows HEX. This keeps the button layout regular while making hex entry discoverable.
 
 All important actions have direct keys (digits, operators, =/Enter, Backspace, Tab for BASE, c/C, etc.). The on-screen buttons are primarily visual + mouse targets.
 

internal/ui/ui.go

@@ -234,6 +234,7 @@ func (a *App) View() string {
 	opKey := keyStyle.Copy().Foreground(lipgloss.Color("63")).Background(lipgloss.Color("235"))
 	clearKey := keyStyle.Copy().Foreground(lipgloss.Color("203")).Background(lipgloss.Color("52"))
 	modKey := keyStyle.Copy().Foreground(lipgloss.Color("214")) // orange-ish for MOD
+	hexKey := keyStyle.Copy().Foreground(lipgloss.Color("214")).Background(lipgloss.Color("235")) // for A-F, only shown in HEX mode
 
 	makeKey := func(label string) string {
 		var st lipgloss.Style
@@ -263,11 +264,29 @@ func (a *App) View() string {
 	row3 := lipgloss.JoinHorizontal(lipgloss.Top, makeKey("1"), spacer, makeKey("2"), spacer, makeKey("3"), spacer, makeKey("-"), spacer, makeKey("AC"))
 	row4 := lipgloss.JoinHorizontal(lipgloss.Top, makeKey("0"), spacer, makeKey("."), spacer, makeKey("+/-"), spacer, makeKey("+"), spacer, makeKey("="))
 
-	// Compact 4-row keypad grid (BASE action is Tab-only; see hint).
+	var rawGrid string
-	rawGrid := lipgloss.JoinVertical(lipgloss.Left, row1, row2, row3, row4)
+	if a.engine.CurrentBase() == calc.BaseHEX {
+		// HEX mode only: A-F row appears at the top of the keypad (only after Tab into HEX).
+		// Use dedicated hexBtn to force hexKey style (avoids label conflict with "C" clear button).
+		hexBtn := func(label string) string {
+			st := hexKey
+			if a.pressedKey == label {
+				st = pressedStyle
+			}
+			return st.Render(label)
+		}
+		hexRow := lipgloss.JoinHorizontal(lipgloss.Top,
+			hexBtn("A"), spacer, hexBtn("B"), spacer, hexBtn("C"), spacer, hexBtn("D"), spacer, hexBtn("E"), spacer, hexBtn("F"),
+		)
+		rawGrid = lipgloss.JoinVertical(lipgloss.Left, hexRow, row1, row2, row3, row4)
+	} else {
+		// Normal mode: standard 4-row grid (no A-F)
+		rawGrid = lipgloss.JoinVertical(lipgloss.Left, row1, row2, row3, row4)
+	}
 
 	// Center the key grid directly under the display.
 	// No enclosing container — just the individually styled keys.
+	// When in HEX the A-F row appears at the top of the keypad.
 	keypad := lipgloss.NewStyle().
 		Width(dispW).
 		Align(lipgloss.Center).