Dive roles: custom roles and the diver's own role on a dive

[ericgriffin]

Closes #551
Closes #547

Summary

Adds a dive_roles reference table (schema v103) mirroring the dive-types pattern: nine seeded built-in roles (the six legacy roles plus Rear Guard, Support Diver, Safety Diver) and user-defined custom roles with UUID ids. The per-dive buddy role now resolves against this table instead of the fixed BuddyRole enum, and dives gain a nullable diver_role column recording the active diver's own role.

#551 - custom/extensible roles

• Role picker sheets list built-ins plus the diver's custom roles, with an "Add custom role..." flow that saves for reuse
• New Dive Roles management page under Settings > Manage Data (add, rename, delete; delete blocked while a role is referenced by any dive)
• Built-in names are localized in all 11 locales (Dutch Rear Guard = "Hekkensluiter", per the issue)
• Unknown role slugs now display as-is instead of silently coercing to "Buddy"

#547 - my own role

• Pinned "Me" chip in the dive edit Buddies card with the same role vocabulary plus a "No role" option
• Dive detail shows a "Me - " row above the buddy list when set
• Consolidation merges adopt diverRoleId first-non-null (follows the Merging dives deletes GPS location data #542 scalar-adopt pattern)

Sync / backup

• dive_roles is an HLC-synced entity: custom rows only (built-ins are seeded locally and re-asserted in beforeOpen), tombstoned deletions, adopt-wipe preserves built-ins
• Custom roles ride in the UDDF full backup with ids preserved so dive_buddies.role / dives.diver_role references resolve after restore
• UDDF interchange export buckets custom roles as plain buddies; leader roles unchanged

Migration

• v103: create + seed dive_roles, add dives.diver_role; idempotent with beforeOpen backstop; existing dive_buddies.role strings match the built-in slugs so no data migration is needed

Testing

• New: migration v103 (4), DiveRoleRepository (9), sync serializer contract (5), selector sheet (5), Me chip (5), management page (5), UDDF backup parse/build (2), merge-builder adopt, repository round-trip
• Updated: buddy repository role resolution, picker widget tests, UDDF importer/exporter fixtures
• Full sweep: database (141), sync (425), export (129), buddies (202), dive_log (1917), settings (608), import wizard (627) all pass; flutter analyze clean

Remaining manual verification

• On-device smoke of the Me chip flow and custom-role creation
• Two-device sync check for custom role replication

[codecov]

Codecov Report

❌ Patch coverage is 92.59797% with 51 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lib/core/database/database.dart	67.56%	12 Missing ⚠️
..._roles/data/repositories/dive_role_repository.dart	92.38%	8 Missing ⚠️
...es/presentation/providers/dive_role_providers.dart	88.70%	7 Missing ⚠️
...tings/presentation/providers/export_providers.dart	0.00%	6 Missing ⚠️
...es/dive_log/presentation/pages/dive_edit_page.dart	62.50%	3 Missing ⚠️
...dive_roles/presentation/pages/dive_roles_page.dart	97.85%	3 Missing ⚠️
...presentation/widgets/dive_role_selector_sheet.dart	96.82%	2 Missing ⚠️
lib/core/router/app_router.dart	50.00%	1 Missing ⚠️
lib/core/services/sync/sync_data_serializer.dart	97.72%	1 Missing ⚠️
...es/buddies/data/repositories/buddy_repository.dart	90.00%	1 Missing ⚠️
... and 7 more

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