Settings: let the user rename the attachments folder

[thetechjon]

Context

Attachments default to Files/ (configurable in code via attachmentsFolder in src/utils/systemFolder.ts) but the Settings UI does not surface this. Some users prefer attachments/, _assets/, or whatever convention their Obsidian vault used — and currently they have to know the internal helper exists to override it.

Mirrors issue #27 (trash folder rename) — same shape, different field.

Where to look

• src/utils/systemFolder.ts — attachmentsFolder helper with .get() / .set().
• src/stores/settingsStore.ts — where the persisted field lives.
• src/components/modals/SettingsModal.tsx — Settings → Vault section, near the trash settings.
• src/utils/attachments.ts — saveAttachment reads attachmentsFolder.get() for new saves.

Acceptance criteria

• Settings → Vault gains a text input "Attachments folder" with default Files.
• Changing the value affects NEW attachments only. Existing attachments at the old path are not moved (rename of existing files is out of scope for this issue — open a follow-up if needed).
• Sanitization: reject empty strings and filesystem-illegal characters.
• The setting persists per vault (same shape as trashMode).

Complexity

Small.

For first-time contributors

Read CONTRIBUTING.md. npm install, npm run dev. PR against dev. If you also tackle the rename-on-update case, mention it in the PR and we can keep both behind one shipped feature.