/claim #5756

What does this PR do?

Adds a standalone Proton Calendar app (app.cal.com/apps/proton-calendar) that handles Proton-specific ICS feed nuances, separate from the generic ICS feed app.

This follows the approach suggested by @PeerRich and @alishaz-polymath — a Proton-specific app based on the ICS feed app where we can handle Proton quirks without breaking the generic ICS app for other providers.

Key improvements over the base ICS app:

• Fix ghost events: Filters STATUS:CANCELLED events that Proton includes in its ICS feeds, which were blocking availability incorrectly
• Fix cancelled recurring occurrences: Tracks cancelled occurrences by UID + recurrence-id so phantom busy slots from cancelled instances of recurring events are properly skipped
• Proton URL validation: Only accepts proton.me / protonmail.com domains (prevents SSRF, ensures correct app usage)
• Proton branding: Dedicated app with Proton Calendar icon, description, and setup instructions
• Default calendar name fallback: Uses “Proton Calendar” when x-wr-calname is missing from the feed

Architecture

The app follows the exact same patterns as the existing ICS feed app:

• _metadata.ts for app registration
• api/add.ts for credential setup (with Proton domain validation)
• lib/CalendarService.ts — based on ICS feed service with Proton-specific fixes
• Read-only by design (Proton does not support third-party writes due to E2E encryption)

How to test

1. Install the Proton Calendar app from the app store
2. In Proton Calendar → Settings → Calendars → Share → create a “Link to calendar” (ICS feed)
3. Paste the URL into the app setup
4. Verify:
   • Cancelled events no longer block availability (ghost events fixed)
   • Recurring events expand correctly within the availability window
   • Non-Proton URLs are rejected

Mandatory Tasks

• I have self-reviewed the code
• N/A — no developer docs changes needed
• Automated tests — happy to add if requested; the CalendarService logic mirrors the existing ICS feed app with targeted Proton fixes

Fixes #5756