Add SSH key authentication for SFTP connections

[anilbeesetti]

Summary

• add SFTP connections with password or SSH private-key authentication
• securely stage, persist, replace, reconcile, and delete imported private keys
• add strict SHA-256 TOFU host-key verification with trusted/presented mismatch details
• add seekable SFTP streaming, Android SSHJ crypto-provider support, Room v8 migration, and connection UI

SSH-key authentication is intentionally scoped to SFTP. SMB, FTP, and WebDAV continue using their existing password authentication.

Fixes #1806

Testing

• ./gradlew test ktlintCheck assembleDebug --rerun-tasks --console=plain — 516 tasks passed
• :feature:network:testDebugUnitTest :feature:network:lintDebug ktlintCheck
• release R8 minification passed
• Room v7-to-v8 migration passed on a disposable emulator
• Android instrumentation passed for X25519, ECDSA host-key decoding, and Ed25519 support without replacing Android's platform BC provider
• disposable API 36 emulator against an ECDSA-only, Curve25519 SFTP server:
  • password reject/trust/save
  • unencrypted Ed25519 key
  • encrypted Ed25519 key with missing, incorrect, and correct passphrases
  • nested browsing and playback launch
  • process relaunch persistence
  • rotated-host mismatch showing both fingerprints with no Trust action
  • key-auth connection deletion

Seek interaction was not meaningful with the short disposable video fixture; playback launch and rendered output were verified.