feat: add whitelist directories for auto-upload

[ClaudiaFang]

Background

Currently the plugin appears to auto-upload every image regardless of which folder it's saved in. Users may want images outside specific note/attachment folders to be left alone (not uploaded to the bucket).

Goal

Add a whitelist setting where the user can specify one or more directories (paths within the vault). Only images saved/referenced under those whitelisted directories should trigger the automatic upload flow; images elsewhere in the vault should be skipped.

Open Questions

• Should the whitelist support glob patterns / subfolder matching, or only exact top-level folder names?
• Default behavior when the whitelist is empty — upload everything (current behavior) or upload nothing until configured?
• Should this be configurable per-vault or is a single global setting sufficient?

[ClaudiaFang]

Backlog priority review — 2026-08-04

Priority: P1

Reason: Unintended automatic uploads can expose private or irrelevant vault images and create storage noise, so scope control is an important product-safety feature rather than cosmetic enhancement.

Scheduling note: Implement on top of the finalized #19 settings architecture. Preserve existing behavior when the list is empty, support normalized folder-prefix/subfolder matching first, and defer complex glob syntax unless users need it.