feat: harden and polish gitt CLI (closes #210)

- Fix bounty precision loss using Decimal conversion with max 9 decimal places
- Add minimum bounty validation (10 ALPHA)
- Validate repository format (regex) and verify existence on GitHub API
- Validate GitHub issue exists, is open, and is not a PR
- Validate on-chain issue IDs are in range (1-999999)
- Validate SS58 addresses with scalecodec fallback to regex
- Extract format_alpha() shared helper replacing 12 inconsistent / 1e9 conversions
- Remove deprecated get_ws_endpoint() function
- Add Rich spinners for all long-running operations (RPC, contract reads, tx)
- Add Rich Panels for all transaction commands (vote, admin, register)
- Color-code status column in issue list (green/yellow/dim)
- Add --json flag to read commands (list, bounty-pool, pending-harvest, vote list, admin info)
- Standardize help text, success/error messages, and wallet option descriptions
- Add print_success/print_error/print_network_header UX helpers

Author: MkDev11