Show a one-time, dismissable CC0 acknowledgment modal informing the user that the workbench persists their drafts/preferences to a public Commons user-subpage, and that this content is therefore licensed CC0 by Commons policy.

Why

The Upload Workbench writes user state to two public JSON pages on Commons:

• User:<self>/UploadWorkbench/Preferences.json — UI prefs, required-field config, custom column defs
• User:<self>/UploadWorkbench/Metadata.json — drafts (per-file edits), filename cache, hidden-file list, history cache, manual photo groups

Both pages are written under the user's own user namespace and are publicly visible / mirrored / archivable like any other Commons page. By Commons convention all wiki contributions are CC BY-SA 4.0 + GFDL by default, but per Commons:Licensing#Material_in_the_public_domain factual data has no copyrightable expression — the maintainer's intent is that this data is treated as effectively CC0 (public-domain dedication). The user should be informed of this once, with an explicit acknowledgment, before the tool starts persisting their data to public pages.

What

Add a small modal that appears the first time a logged-in user lands on the workbench. The modal:

• Explains briefly that drafts and preferences are saved to a public page in the user's user namespace.
• States clearly that this content is, by intent of the maintainer, dedicated as CC0.
• Provides a link to the user's own User:<self>/UploadWorkbench/ so they can inspect what's stored.
• Has two buttons:
  • "I agree — remind me next session" → records the acknowledgment but reprompts on next session.
  • "I agree — don't remind me again" → records the acknowledgment permanently; modal never appears again.
• The modal blocks no functionality (the user can dismiss with either button or Esc), but it appears before any user-store write happens for a fresh user.
• Pressing Esc / backdrop click is treated as no acknowledgment (modal will reappear on the next session) — explicit choice required to suppress.

Where

• Persistence: extend Preferences.json with a single new key: ` cc0Acknowledgment: { acknowledgedAt: <ISO>, suppressFurther: <bool>, version: 1 } | null ` Read/write via the existing getPref / setPref API in src/api/user-store.js:766-779 (just-add-a-key — no schema migration).
• Trigger: mounted in <App> (src/app.jsx:1932-1977) alongside the existing <InfoModal>. Decision logic:
  • If cc0Acknowledgment is missing or null → show modal on mount.
  • If cc0Acknowledgment.suppressFurther === true → never show.
  • Otherwise (acknowledged but not suppressed) → show on every fresh page load (per-tab is fine; the wiki write happens on click, the in-memory render decision is per-tab).
• New UI file: src/ui/cc0-modal.jsx (regular ESM import, follows the wikitext-preview-modal.jsx pattern — small, self-contained, uses existing .modal-backdrop / .modal styles from src/app.css:3631-3682).

Acceptance

• Fresh login (no cc0Acknowledgment pref) → modal appears on first paint.
• Click "remind me next session" → modal disappears; reload page → modal reappears.
• Click "don't remind me again" → modal disappears; reload page → modal does not reappear.
• Esc / backdrop click → modal disappears; reload page → modal reappears.
• DEMO_MODE → modal does not appear (no auth, no wiki writes).
• Modal copy is in English (the rest of the UI is too) and the rendered Preferences.json contains the new field after either click.
• npm run build (incl. scripts/check-undefined-refs.mjs) passes.

Notes

• Pre-existing users (anyone who has used the tool before this MR lands) will see the modal once — this is intentional. They retroactively get the chance to acknowledge.
• Storage shape includes a version field so a future copy/scope change can re-prompt by bumping the version number (the check becomes ack.version === 1 && ack.suppressFurther — out-of-date version reprompts).