Remove dead code: TPEN.getAllPublicProjects() in TPEN.js

## Cleanup

**Type:** Dead code removal
**File:** `api/TPEN.js`

### Description

`getAllPublicProjects()` (lines 168-172) is dead code — never called from any component, page, or route in the codebase. It fetches `GET /projects/public`, a route that doesn't exist on the backend either.

### Dead Code to Remove

```javascript
async getAllPublicProjects() {
// Logic to fetch all public projects
return fetch(`${this.servicesURL}/projects/public`)
.then(response => response.json())
}
```

Also remove any references in `_classes/TPEN.md` documentation (lines ~20, 80-82).

### Context

Found during pre-production stack testing (2026-03-26). The backend route was flagged as missing (CenterForDigitalHumanities/TPEN-services#496), but investigation confirmed nothing in Interfaces ever calls it. Services issue closed as not_planned.

This appears to be a stub for a "browse public projects" feature that was never implemented. If this feature is planned for the future, a new issue should track both the backend route and the frontend UI together.