#85 Add multi-account config struct and per-account storage layout

Storage model change: ~/.config/opake/ gets accounts/<sanitized-did>/ subdirs. DID sanitization: replace ':' with '_'. New Config struct: default_did + BTreeMap<String, AccountConfig { pds_url, handle }>. Add account_dir(did), save_account_json/load_account_json helpers. Config format is TOML with [accounts."did:plc:xxx"] tables.