docs: update project rules for demo angle modes and UI structure

.continue/rules/project.md

@@ -31,14 +31,33 @@
 - `sqrt`, `asin`, `acos`, `log`, and `ln` must throw clear, explicit domain errors
 - `log` uses `Math.log10`
 - `ln` uses `Math.log`
-- Trigonometric functions use degrees in the demo:
-  - `sin`, `cos`, `tan` convert degrees to radians
-  - inverse trig functions return degrees
+- Trigonometric functions must support `deg`, `rad`, and `grad`
+- In the browser demo, degrees are the default angle mode
 - `inputValue` must remain a string to preserve future hexadecimal input support
+
+## Demo rules
+
+- The browser demo lives under `samples/dev/`
+- `samples/dev/index.html` is the demo entry page
+- `samples/dev/index.css` contains the calculator visual theme
+- `samples/dev/index.js` contains demo-side presentation helpers and UI logic
+- The demo UI must expose:
+  - a stack display
+  - a main display
+  - a visible angle mode indicator
+  - an angle mode selector for `deg`, `rad`, and `grad`
+- The demo may present user-facing labels such as `+`, `−`, `×`, `÷`, `y^x`, `1/x`, and `x²` while still using English command identifiers internally
 - The example HTML must group buttons by `Stack`, `Arithmetic`, and `Trigonometry`
 - The example HTML must call `command(...)` for actions
+- Keyboard support in the demo should remain consistent with the displayed help text
+
+## File references
+
+- `samples/dev/index.html` references `src/rpn-calculator.js` as the calculator engine used by the demo
+- Keep the demo aligned with the public calculator API exposed by `src/rpn-calculator.js`
 
 ## Maintenance
 
 - Keep this file updated after each project change using the provided editing tools.
+- When changing the demo UI or calculator API, update these rules so they continue to match the current project behavior.