Filebrowser: editable, pasteable path input

[Elompenta]

Summary

• Turns the file browser's current-directory display into an editable input: type or paste a path and press Enter to jump the tree there.
• Strips surrounding quotes from Windows Explorer's "Copy as path" so a pasted path works directly.
• Reverts to the last valid path on invalid input, showing the server's error in the file list.

Closes #50

Test plan

• Click the path field, verify it left-aligns and selects the full path for easy overwrite/paste
• Type/paste a valid path and press Enter — tree jumps to that folder
• Paste a quoted path ("C:\...\folder") — quotes stripped, tree jumps correctly
• Enter an invalid path — shows "Not a directory", input reverts to last valid path, navigation still works (↑ button, clicking folders)