feat(preferences): configurable photo info attributes stored in user profile

- backend: users.preferences jsonb (migration 000004), Me returns preferences, PATCH /auth/me/preferences with validation (visible list + labels)
- frontend: gear in info section opens PhotoInfoSettingsDialog; base attribute set shown by default, remaining under ellipsis; per-attribute visibility and custom labels persisted to the profile; reset clears settings from the profile
- frontend: dialog has select-all/deselect-all icon buttons (no 'show all'); EXIF constants extracted to lib/exif.ts
- fix: render modal overlays via portal to document.body so they center in the viewport despite transformed panel/rail ancestors
- openspec: sync lightroom-workspace-ui artifacts (spec/design/proposal/tasks 9.8)

backend/migrations/000004_user_preferences.down.sql

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+ALTER TABLE users DROP COLUMN IF EXISTS preferences;

backend/migrations/000004_user_preferences.up.sql

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+ALTER TABLE users ADD COLUMN IF NOT EXISTS preferences jsonb NOT NULL DEFAULT '{}'::jsonb;