Adds a non-destructive, dismissable banner on the Dashboard that invites
profiles still on the legacy v2 category seed to discover the new v1 IPC
taxonomy. The banner links to the standard categories guide page
(/settings/categories/standard, shipped in #117).
Visibility rules:
- Only rendered when `categories_schema_version='v2'` in `user_preferences`.
- Hidden once the user dismisses it — dismissal is persisted in the same
`user_preferences` table under the key `categories_v1_banner_dismissed`
(value '1'), so the banner never reappears across app restarts.
- New v1-seeded profiles never see it.
No DB schema change: reuses the existing key/value `user_preferences`
table via the existing `getPreference`/`setPreference` helpers. No
migration added.
i18n keys under `dashboard.categoriesBanner.*` (FR + EN).
Changelog entry added under [Unreleased] in both CHANGELOG.md and
CHANGELOG.fr.md.
