Let me break down the requirements:

1. A heptagon (7-sided polygon) that spins 360° every 5 seconds
2. 20 numbered balls (1-20) dropping from the center of the heptagon
3. Balls are affected by gravity and friction
4. Balls collide with each other (ball-ball collision)
5. Balls bounce realistically off the rotating walls (ball-wall collision with the spinning heptagon)
6. Each ball shows its number, and the number rotates (spins) based on the ball's rotation
7. All collision detection implemented manually (no physics library)

This is a physics simulation. I need to implement:
- Heptagon geometry (7 vertices, rotating)
- Gravity (constant downward force)
- Friction (velocity damping or surface friction)
- Ball-ball collision detection and response (circle-circle)
- Ball-wall collision detection and response (circle-segment, where segments are the heptagon edges)
- Ball spin/rotation visualization
- All in a single HTML file with inline CSS and JS

Let me think about the physics:

**Heptagon:**
- 7 vertices evenly spaced around a circle
- Rotates 360° every 5 seconds → angular velocity = 2π/5 rad/s
- Need to compute vertex positions at each frame based on rotation angle

**Ball physics:**
- Position (x, y), velocity (vx, vy)
- Angular velocity (spin)
- Gravity: vy += g * dt
- Friction: v *= (1 - friction * dt) or angular friction
- Air resistance / damping

**Ball-ball collision:**
- Check distance between centers < sum of radii
- Resolve overlap by pushing apart
- Exchange velocity components along collision normal
- Apply restitution coefficient

**Ball-wall collision (rotating walls):**
- For each edge of the heptagon, check if ball is outside (or too close)
- The walls are moving, so wall velocity at contact point needs to be accounted for
- Reflect ball velocity relative to wall velocity
- Need to find the closest point on each line segment to the ball center
- If distance < ball radius, push ball inside and adjust velocity

Let me think about the coordinate system. Center of canvas, heptagon centered there. Balls start at center and fall due to gravity. Gravity points downward (positive y if y increases downward, as in canvas).

For ball-wall collisions with rotating heptagon:
- Each wall is a line segment between two consecutive vertices
- The vertices rotate, so walls move
- At a point on the wall, the velocity due to rotation is: v = ω × r (perpendicular to the radius)
- When ball hits wall, compute relative velocity of ball minus wall velocity at contact point
- Reflect the normal component of relative velocity (with restitution)
- Apply friction in tangential direction

For the spin visualization:
- Each ball has an angular position (draw angle)
- The number is drawn rotated by this angle
- Ball spin changes based on friction when contacting walls or other balls

Let me code this up. I'll use a canvas-based approach.

Let me think about the heptagon size. If the canvas is say 800x800, the heptagon could have a radius of about 300 pixels. Balls could have radius around 15-18 pixels.

For 20 balls in a heptagon with radius 300, that should fit reasonably.

Let me code this:

```javascript
// Heptagon setup
const numSides = 7;
const heptagonRadius = 300;
const angularVelocity = (2 * Math.PI) / 5; // 360° per 5 seconds

// Ball setup
const numBalls = 20;
const ballRadius = 15;
const gravity = 500; // pixels/s²
const restitution = 0.7;
const friction = 0.98; // velocity damping per frame (or per second)

// State
let heptagonAngle = 0;
let balls = [];

// Ball object: {x, y, vx, vy, radius, number, angle, angularVel}
```

For ball-wall collision with rotating walls, I need to be careful. Let me think through this:

1. Compute the 7 vertices of the heptagon at current angle
2. For each edge (segment from vertex[i] to vertex[(i+1)%7]):
   - Find closest point on segment to ball center
   - Compute distance
   - If distance < ballRadius:
     - Ball is penetrating the wall
     - Normal = (ball center - closest point) / distance
     - Push ball out: move ball center along normal
     - Wall velocity at contact point: v_wall = angularVelocity × r_contact (tangent to circle centered at origin)
     - Relative velocity = ball_velocity - wall_velocity
     - Normal component of relative velocity
     - If approaching (normal component < 0):
       - Apply impulse in normal direction (with restitution)
       - Apply friction impulse in tangential direction
       - Update angular velocity based on tangential friction

Let me implement this properly.

For a point rotating around the origin, velocity = (-ω * y, ω * x) where ω is angular velocity.

Let me write the full code now.

Actually, let me reconsider the ball drop. "Dropping from the center of a heptagon" - they start at center, maybe with slight random spread, and fall due to gravity. Since the heptagon is spinning, the balls will bounce around inside it.

Let me also think about the spinning number visualization. Each ball has a number drawn on it. The number should rotate with the ball's spin. I'll draw the ball as a colored circle with the number rotated by the ball's angular position.

Let me write this all out now.

I need to be careful with the ball-wall collision for a rotating container. The key insight is:
- The walls are not static - they rotate
- When a ball hits a wall, we need to consider the wall's velocity at the contact point
- The wall velocity at a point (px, py) on the wall is (-ω*py, ω*px) where ω is the angular velocity

Let me also think about the friction model for ball-ball collisions. When two balls collide, the tangential component of velocity at the contact point can transfer some spin. But for simplicity, I can just have ball-ball collisions affect linear velocity and slightly affect angular velocity.

For ball-wall collisions, I'll update angular velocity based on the friction impulse.

Let me write it all now.

One more consideration: the heptagon interior. Since the balls are inside, I need to make sure balls stay inside. The normal for collision should point inward (from wall toward center).

For finding the closest point on a segment: project the ball center onto the line, clamp to segment endpoints.

Let me also add some color variety to the balls for visual appeal.

Let me think about the initial conditions. 20 balls dropping from the center - they should start at slightly different positions (staggered) so they don't all overlap. Maybe in a small grid or random positions near center.

Actually, let me stagger them vertically with small random offsets so they naturally fall and spread out.

OK let me write the complete HTML file now.

Let me verify the file was created correctly.