feat: add MultiSelect and Url custom field types with corresponding editors

[pikann]

Summary

The backend's FieldType enum supports 7 custom field types (text, number, date, select, multi_select, boolean, url), but the web app only had editors for 5. multi_select and url fields could only be created by calling the API directly — there was no UI path to create them — and once created, their values were silently mangled (a multi_select array like ["backend", "bug"] got passed through String() and rendered as "backend,bug").

This PR adds full UI support for both types, end to end: creation, display, and editing.

Changes

• New editors: MultiSelectEditor (chip-based multi-pick over fixed options) and UrlEditor (renders as a clickable external link when read-only, falls back to plain text if the value isn't a real http(s):// URL)
• Type mapping: helpers.ts's API_TO_UI_FIELD_TYPE / UI_TO_API_FIELD_TYPE are now a lossless 1:1 mapping instead of collapsing multi_select → Select and url → Text
• Field creation: both the in-task-detail "Add field" dialog and the project settings custom fields manager can now create MultiSelect/Url fields, including an options editor for MultiSelect (reusing the existing Select options UI)
• i18n: added multiSelect/url labels and a multiSelectEditor.addOption string to all 7 locales

Testing

• tsc -b and biome check pass clean across apps/web
• Manually created a multi_select field, set values via the new editor, confirmed display/edit round-trips correctly