feat(editor): Save As dialog on Ctrl+Shift+S

[sspaeti]

Why this PR: Usually, saving to the default location is enough. But I have some screenshots that I take for certain things - then it's handy to save them directly there, or replace an existing image with a new, better one.

So here's an easy Save as option. The harder part was working around the overlay image, so it hides Omasnap when you choose the file location and opens back up after saving, so you can close it afterward, save to the default spot, or save to the clipboard.

Claude Fable Summary

• Adds a Save As action (Ctrl+Shift+S) in the editor: opens a file dialog to pick a destination and writes a copy there.
• The editor stays open afterwards — Enter/Ctrl+C/Ctrl+S default outputs are unchanged.
• The chosen folder is remembered (output/save_as_dir in the config) and offered as the start directory next time.
• The fullscreen stays-on-top overlay is hidden while the dialog is up so the dialog isn't buried behind it, then restored.
• Listed in the hotkey legend and README.

Test plan

• make build and lint pass.
• Manual: capture, annotate, Ctrl+Shift+S, pick a path — file written, editor still open, next Save As starts in the same folder. Cancel path leaves editor usable.