feat(calendar): add vdirsyncer and local vdir calendar support

[xela-zone]

Summary

Adds support for local vdir and vdirsyncer calendar directories to Noctalia's calendar service:

• vdir_reader module: Depth-bounded recursive collection discovery capable of resolving nested folder hierarchies (e.g. ~/. local/share/calendars/<account>/<collection>/).
• Metadata parsing: Reads standard vdir metadata files (displayname, color, order) and X-WR-CALNAME properties with fallback to folder names.
• Inotify event monitoring: Watches discovered collection directories for .ics file creations, modifications, and deletions with a 300ms debounce
  directly on the main poll loop.
• Settings GUI: Added a Local (vdir) provider option and directory path picker/validator in Settings → Services → Calendar.
• Unit tests: Added unit tests in tests/vdir_reader_test.cpp.

Motivation

I personally already have my caldav synced using vdirsync, to allow one syncing tool to work with multiple tools, such as khard or even someone using a local only calender app.

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Testing

• Added unit tests in tests/vdir_reader_test.cpp covering:
  • Nested Collection Discovery: Recursive traversal across multi-level directory trees (e.g. <account>/<collection-uuid>/).
  • Direct Collection Discovery: Resolving accounts pointed directly at a single collection folder.
  • Directory Filtering: Ignoring hidden directories (e.g. .git/), temporary directories (.tmp), and non-directory files.
  • Metadata File Parsing: Correct extraction and trimming of displayname, color, and order files, plus collection sorting by order.
  • X-WR-CALNAME Fallback: Extracting calendar titles directly from .ics file headers when no displayname file is provided.
  • Event Loading & Mapping: Parsing .ics events from discovered collections with proper mapping of IDs, summaries, calendar names, and color
    assignments.

Manual Coverage

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

after creation / edit panel:

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I will update end-user documentation after merge, or this PR does not change user-facing configuration or behavior.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.

Additional Notes

written by gemini / antigravity, reviewed and lead by a human