feat(calc-02): add shared popup menus for mode and constants

Refactor the calc-02 demo to use a single popup menu component for angle mode and constants, align the menus to their trigger buttons, and update the README/project notes to reflect the portrait-first demo layout and constant API.

README.md

@@ -16,15 +16,15 @@ The main class is `RpnCalculator`.
 ## Project structure
 
 - `src/rpn-calculator.js`: calculator engine
-- `samples/dev/index.html`: active browser demo entry point
+- `samples/dev/index.html`: browser demo entry point
 - `samples/dev/index.css`: demo styles
 - `samples/dev/index.js`: demo UI and keyboard logic
 - `samples/calc-01/index.html`: active browser demo entry point
 - `samples/calc-01/index.css`: demo styles
 - `samples/calc-01/index.js`: demo UI and keyboard logic
-- `samples/calc-02/index.html`: new responsive HP48GX-style demo entry point
-- `samples/calc-02/index.css`: new responsive demo styles
-- `samples/calc-02/index.js`: responsive HP48GX-style demo UI and keyboard logic
+- `samples/calc-02/index.html`: portrait-first HP48GX-style demo entry point
+- `samples/calc-02/index.css`: portrait-first demo styles
+- `samples/calc-02/index.js`: portrait-first HP48GX-style demo UI and keyboard logic
 - `samples/calc-XX/`: placeholder name for future demo variants
 
 ## Public API
@@ -45,6 +45,10 @@ Instance helpers also available:
 
 - `getOperationsByCategory()`
 - `getConstants()`
+- `listConstants()`
+- `setConstant(name, value)`
+- `removeConstant(name)`
+- `hasConstant(name)`
 
 Static helpers also available:
 
@@ -84,6 +88,7 @@ Available constants:
 
 - `pi`
 - `e`
+- plus any user-defined constants added through the engine API
 
 They can be used through `command(...)`:
 
@@ -246,6 +251,11 @@ Main UI features:
 - grouped panels for `Stack`, `Arithmetic`, `Trigonometry`, and `Constants`
 - error display area
 
+## Calc 02 demo
+
+`samples/calc-02/` is a portrait-first HP48GX-inspired demo.
+It keeps the display-adjacent button row aligned in four columns, uses compact popup menus for mode and constants, and supports clipboard paste plus the `y√x` root operation.
+
 The demo loads the engine from:
 
 ```html README.md
@@ -270,11 +280,6 @@ The current demo supports:
 The demo also implements stack selection and stack-item move mode in its UI layer using the public calculator methods.
 It keeps the calculator screen focused and updates the visible stack window as the selection moves.
 
-## Calc 02 demo
-
-`samples/calc-02/` is a responsive HP48GX-inspired demo.
-It adapts its layout to the browser window, supports portrait and landscape arrangements, and includes the `y√x` root operation.
-
 ## Exports
 
 `RpnCalculator` is exposed in both environments: