feat: add bills command group (list, download)

[datenwurm]

Description:
Add the /bills resource of the Cantamen casirest/v3 API: bills list shows bill summaries newest first with the accumulated total per currency and a --year filter; bills download fetches a bill's itemized PDF via the expanded billDocument (inline base64 or reference URL). Bill line items only exist inside the PDF, so list totals are bill-level by design.

Example:

$ teilauto bills
                                                                                                                                           
 Usage: teilauto bills [OPTIONS] COMMAND [ARGS]...                                                                                         
                                                                                                                                           
 List your bills and download their PDFs (login required).                                                                                 
                                                                                                                                           
╭─ Options ────────────────────────────────────────────────────────────╮
│ --help  -h  Show this message and exit.                              │
╰──────────────────────────────────────────────────────────────────────╯
╭─ Commands ───────────────────────────────────────────────────────────╮
│ download  Download the itemized PDF for BILL_ID.                     │
│ list      Show your bills, newest first, with the accumulated total. │
╰──────────────────────────────────────────────────────────────────────╯

$ teilauto bills list --help
                                                                        
 Usage: teilauto bills list [OPTIONS]                                   
                                                                        
 Show your bills, newest first, with the accumulated total.             
                                                                        
╭─ Options ────────────────────────────────────────────────────────────╮
│ --year      YYYY  Only bills from this year.                         │
│ --json            Output machine-readable JSON.                      │
│ --help  -h        Show this message and exit.                        │
╰──────────────────────────────────────────────────────────────────────╯

$ teilauto bills list
                   Bills                    
┏━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━━━┓
┃ ID       ┃ Number  ┃ Date       ┃ Amount     ┃
┡━━━━━━━━━━╇━━━━━━━━━╇━━━━━━━━━━━━╇━━━━━━━━━━━━┩
│ 12345678 │ abcdefg │ 2026-07-01 │  56.64 EUR │
└──────────┴─────────┴────────────┴────────────┘
        1 bill(s) · total 56.64 €     

$ teilauto bills download 12345678
✓ Saved bill-12345678.pdf (111192 bytes)

Notes:
The endpoint was mapped statically from the official Android app (same backend, same casirest/v3 base) rather than the booking website.