fix(freeze): add portable Wayland capture fallback

[devmobasa]

Closes #156.

Summary

Improve Freeze capture support across Wayland compositors, including COSMIC and compositors without
wlr-screencopy.

Freeze now tries capture backends in this order:

1. wlr-screencopy
2. ext-image-copy-capture-v1
3. Screenshot portal

Changes

• Add native ext-image-copy-capture-v1 support with SHM capture, output transforms, constraint replacement, bounded retries, and protocol cleanup.
• Fall back across backends after startup errors, asynchronous failures, or three-second direct-capture timeouts.
• Preserve the initiating output identity and discard results if the active output changes before capture or activation completes.
• Fix the portal response race by subscribing before calling Screenshot.
• Generate secure 128-bit portal request tokens and handle replacement request paths.
• Detect portal availability through the Screenshot interface version.
• Preserve portal response code 1 as quiet user cancellation.
• Validate compositor-provided SHM dimensions, stride, formats, padding, and buffer lengths before allocation or protocol submission.
• Remove screenshot URIs and temporary paths from logs and errors.
• Update capability reporting and project documentation.