Use a folder picker for watched folder settings

[bobthearsonist]

Problem

Visual Notes watched folders are currently entered as free-text paths. That makes setup error-prone because users have to type or copy exact vault-relative folder names.

Proposal

Replace or augment the watched-folder text inputs with an Obsidian folder picker/suggest control.

Acceptance criteria

• Users can choose watched folders from existing vault folders.
• The saved value remains a vault-relative folder path.
• Manual entry is still possible if needed for advanced cases.
• Missing-folder validation still warns clearly.
• Keyboard navigation/accessibility works for the picker.