docs: add on-chain event schema documentation (#109)

[Philzwrist07]

Closes #109

Summary

Both contracts emit Soroban events on every state transition, but there was no documentation of
the topic names, data fields, or encoding format, making it impossible for off-chain indexers
and frontends to reliably parse them.

This PR adds the missing documentation and TypeScript types to fully address that gap.

Changes

docs/events.md

• Documents all 18 events emitted by the governance and token contracts
• Each entry includes: event name, topic symbols, data fields with types and descriptions, and
  a JSON example
• Covers encoding format (XDR ScVal types for Address, i128, u64, String, and enum variants)
• Ends with a quick-reference table for all events at a glance

frontend/src/events.ts

• TypeScript interface for every individual event
• GovernanceEvent, TokenEvent, and CosmosVoteEvent union types for exhaustive pattern matching
  in consumers
• GOV_TAGS and TOKEN_TAGS constants to avoid magic strings when filtering events

Testing

Documentation-only change. No contract logic was modified.

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