Fix/issues 7 8 9 expense ledger improvements

[Arun-kushwaha007]

This PR adds group ledger search/filtering, expense categories, and saved default split preferences for
groups.

Changes

• Added client-side ledger search by description, payer, amount, and date
• Added payer, category, and date-range filters with reset support
• Added optional expense categories across shared types, backend, and web
• Added category selection in the create expense modal
• Added category display in expense rows
• Added group-level saved default split preferences
• Added backend support to store and return default split preferences
• Prefilled the create expense modal from the saved group default split
• Added quick reset back to equal split

Verification

• pnpm --filter backend prisma:generate
• pnpm --filter backend lint
• pnpm --filter web lint
• pnpm --filter backend test

Commits

• feat(expenses): add categories and saved group split defaults (#8, #9)
• feat(web): add group ledger search and filters (#7)

If you want the cleanest GitHub style, change the first line to:

Closes #7
Closes #8
Closes #9