Add batch member confirmation system (preregistering members)

[georgelgeback]

Changes:

• Adding batch members with multiple different identifiers
• New tab pre-filtered on this to automatically accept members who have been pre-registered

Requires the following backend PR to be merged before tests have any chance to pass:

• Batch confirm members WebWebWeb#549

[maffe03]

Från copilot (local):

Potential Areas to Review:

• The batch input format (complex vs. simple) is inferred from semicolon presence—ensure this is intuitive for end users
• Phone number validation uses Swedish region—confirm this is the desired behavior
• Consider adding unit tests for handleBatchAdd validation logic
• The STIL ID regex pattern (/^[a-z]{2}\d{4}[a-z]{2}-s$/) should be documented or explained

Recommendation
Ready for review with the understanding that backend integration is pending. All frontend type checking passes and the implementation is well-structured.

[maffe03]

LGTM