Improve note encryption with per-note KDF salt

[pbek]

Note encryption should stop relying on legacy weak key derivation/static salt behavior and move to a portable, self-contained encrypted-note format.

Requested changes:

• Add a versioned encrypted note envelope that stores non-secret metadata with the ciphertext.
• Store a per-note/per-encryption salt in the encrypted note block so synced notes remain portable across machines.
• Use Botan PBKDF2 with stored parameters for new encrypted notes; the exact PRF should match what the bundled/system Botan builds support portably.
• Use authenticated encryption semantics for new encrypted notes, e.g. an authenticated mode where available or encrypt-then-MAC with a random IV when using the bundled Botan feature set.
• Keep legacy encrypted notes decryptable and migrate them by re-encrypting with the new format on save/edit where possible.
• Avoid YAML/frontmatter for crypto metadata; the encrypted block should be self-contained.

The synced Markdown file should remain the source of truth so Nextcloud Sync continues to work without local database metadata.

[pbek]

26.5.3

• New note encryption now writes a portable versioned encryption envelope with
  per-note salt and nonce metadata, derives keys with Botan PBKDF2-HMAC-SHA1,
  and encrypts with AES-256-CBC plus HMAC authentication while keeping legacy
  encrypted notes decryptable (for #3598)