Redact SmbConfig password and centralize default SMB port

[in-jun]

Two small hardening/hygiene fixes to SmbConfig and its call sites.

• Override toString() on SmbConfig so the plaintext password is rendered as *** instead of the raw value. The data-class default would dump the secret verbatim, and since withoutSecrets() and a redacting SyncPair both stringify configs, a future debug log or interpolated exception message could leak the password to logcat without anyone noticing in review.
• Add const val DEFAULT_PORT = 445 to SmbConfig's companion and reference it from the four fallback sites (SetupScreen seed + build, ConfigRepository JSON parse + legacy migration) so the default lives in one place instead of five copies.

Fixes #77
Fixes #83

[in-jun]

Both fixes look correct and complete.

#77 — password redaction: The custom toString() covers exactly the constructor properties in declaration order and matches the default data-class format, so the only behavioral change is masking password. Since SmbConfig is the sole RemoteConfig implementor holding a secret and SyncPair's auto-generated toString() delegates to remote.toString(), both the direct-config and enclosing-pair leak paths described in the issue are now closed. equals/hashCode are untouched, so the withoutSecrets() equality semantics still hold. The empty-password case rendering password= (rather than ***) is fine — there's no secret to leak there.

#83 — centralized default: DEFAULT_PORT = 445 is referenced from all four production fallback sites (SetupScreen seed + build, ConfigRepository JSON parse + legacy migration) plus the constructor default. The remaining literal in SmbRemoteStorageIntegrationTest is test-only configuration and correctly out of scope.

No regressions or scope creep. LGTM.