Extract rota display into reusable partials

[peterdrier]

Summary

• Extract inline rota rendering from Views/Shifts/Index.cshtml into three shared partials: _RotaHeader.cshtml (name, badges, tags, description), _BuildStrikeRotaTable.cshtml (all-day shift table with collapsible date ranges and range signup), and _EventRotaTable.cshtml (timed shift table with individual signup)
• Add typed view models (RotaHeaderViewModel, BuildStrikeRotaTableViewModel, EventRotaTableViewModel) for clean partial contracts
• Reduce volunteer browse view from 586 to 259 lines with no visual or behavioral changes

Issues

• Closes Tech debt cleanup: consolidate patterns and remove misplaced responsibilities #198

Test plan

• Browse /Shifts — verify rota headers display correctly (name, priority badges, period badges, tags, visibility, description, practical info)
• Verify build/strike rotas show all-day shift table with collapsible date ranges
• Verify event rotas show timed shift table with signup buttons
• Test range signup form on build/strike rotas
• Test individual signup on event rotas
• Verify date range expand/collapse still works (chevron toggle)
• Verify Signed up / Full / Needs help badges display correctly
• Confirm no visual differences from before the refactor