Improve file browser performance and SSH workflows

[matchaboar]

Summary

• add a performant local and SSH-backed file browser with Git change/last-edited metadata and configurable display settings
• support SSH profiles, remote document/image caching, $HOME fallback when a configured remote directory is missing, and compatibility migration for existing remote annotations
• restore standard macOS title-bar double-click zoom/minimize behavior and harden remote PDF/image refresh paths
• add logic coverage reporting, shell-level SSH fallback tests, and light/dark/empty/error screenshot regression baselines to CI

Test plan

• ./run-tests.sh
• ./run-screenshot-tests.sh
• ./build.sh

[matchaboar]

Closing this fork-local PR; the intended upstream PR is nonatofabio#43.