Import/export, branding options and Pro feature gating #43

Problem

Several commercial polish and adoption features are missing:

• Teams moving from another tool (Canny, Trello, GitHub Issues, CSV) have to re-enter every item manually.
• The board offers only CSS variable overrides; non-developers can't brand it.
• Free vs Pro packaging has no technical mechanism yet.

Proposal

1. Import

• NiroRoadmap -> Import wizard with source adapters:
  • CSV (title, description, status, product, tags, votes, date) with column mapping and a dry-run preview.
  • Canny export (JSON/CSV).
  • Trello board JSON export (lists -> statuses, cards -> items, labels -> tags).
  • GitHub issues (repo + token, labels -> tags, state -> status, reactions -> votes optional).
• Creates statuses/products/tags that don't exist, skips duplicates by title+source ID, batches in AJAX chunks with progress, and logs errors.
• Matching export (CSV/JSON) for backups and migration out (trust builder).

2. Branding and appearance

• Visual settings: accent, radius, fonts inherit, density (compact/comfortable), light/dark/auto, card shadow style, column width.
• Custom CSS field with live preview.
• Optional "Powered by NiroRoadmap" link: on by default in free, removable in Pro.
• Translatable custom labels (button texts, "Nothing here yet", etc.) without .po files.

3. Licensing / feature gating (foundation)

• Central niroroadmap_is_pro() and niroroadmap_feature_enabled( $key ) helpers so free and Pro features gate through one place; Pro features appear in the UI as disabled with an upsell, not hidden.
• Pick licensing provider (Freemius / custom) - decision needed, create a follow-up ADR.
• Everything in the free plugin that WordPress.org guidelines require stays free; no remote calls without opt-in.

Acceptance criteria

• CSV import round-trips with the CSV export (import -> export -> identical data).
• Dry-run shows exactly what would be created/skipped; nothing is written until confirmed.
• Imports of 2,000 items complete without timeouts (chunked) and can be resumed.
• Trello and Canny adapters have fixtures and tests.
• Imported content is sanitized like normal submissions.
• Branding settings output via CSS custom properties; invalid CSS values are sanitized.
• Feature-gating helper has a single filterable source of truth and tests.
• "Powered by" link is rel="noopener", toggleable, and compliant with wp.org rules (not forced for the free version if it conflicts with the guidelines - verify before shipping).

Priority rationale

Low. Commercial/packaging work; schedule once the main feature set is stable. Split into separate issues when scheduled (Import/Export, Branding, Licensing).