Add PDF versioning for repeated exports of the same note

[Dev-Ray-Hunt]

Goal

When the user exports the same note more than once, instead of silently overwriting the previous PDF, implement a versioning system so previous exports are preserved.

Problem

Currently exporting `My Note.md` always writes `My Note.pdf`, overwriting any prior export with no warning. Users lose previous versions with no way to recover them.

Options to Research & Evaluate

Option A — Timestamp suffix

`My Note.pdf` → `My Note_2026-03-25_143200.pdf`

• Simple, always preserves history
• Vault fills up fast if user exports frequently

Option B — Numbered suffix

`My Note.pdf` → `My Note_v1.pdf`, `My Note_v2.pdf`, …

• Clean and human-readable
• Requires scanning the output folder to find the next version number

Option C — Overwrite with backup

Always write `My Note.pdf`, but move the previous version to `My Note_prev.pdf` (keep only 1 backup)

• Keeps the vault tidy
• Only one undo step available

Option D — Setting-controlled behaviour

Let the user choose in Settings: Overwrite / Timestamp / Numbered

Acceptance Criteria

• Research complete — decision made on versioning strategy
• Setting added (if Option D chosen)
• Export correctly versions files per chosen strategy
• Output folder doesn't silently grow unbounded (retention policy if needed)

Notes

• Research whether PDF metadata (author, creation date, version) is a better place to store version info than the filename
• Consider how versioning interacts with the output folder setting

Workflow

• Research & decide on strategy
• Code written by Claude
• Claude explains the new code to Brandon
• Brandon tests per checklist
• Approved — close issue