feat(recovery): single-use recovery code for forgotten master password

In a zero-knowledge vault, forgetting the master password normally
means losing the data — the AES key is derived from the master pw
and the server can't help. This commit adds the standard escape
hatch: a one-time recovery code that key-wraps the AES key so the
user can get back in.

Threat model
============
The plaintext recovery code is shown to the user exactly once, at
generation time. Server only ever stores SHA-256(code) + an AES-GCM
wrap of the vault key under a KEK = PBKDF2(code, kdf_salt, 600k).
Without the plaintext code the server cannot unwrap. The code is
high-entropy (96 bits from a 32-char ambiguity-free alphabet, in 4
groups of 4) — printed form is misreading-resistant.

Single use: redeeming deletes the row inside the same DB.Lock the
lookup happened in, so concurrent redeem attempts are race-free.
Failed redemptions feed both the per-IP rate limit AND the per-
username lockout, so brute-forcing the code is infeasible.

Schema
======
recovery_keys (
  user_id      INTEGER PRIMARY KEY (1:1 with users, FK cascade),
  code_hash    TEXT NOT NULL          (SHA-256 hex of plaintext code),
  kdf_salt     TEXT NOT NULL          (PBKDF2 salt for KEK derivation),
  wrapped_key  TEXT NOT NULL          (base64 AES-GCM ciphertext of vault key),
  wrapped_iv   TEXT NOT NULL          (base64 12B IV for the wrap),
  created_at   DATETIME DEFAULT CURRENT_TIMESTAMP
)

Backend: new unit PM.Handler.Recovery
=====================================
  GET    /recovery-key/status   (auth)         -> { configured, created_at? }
  POST   /recovery-key/setup    (auth + CSRF)  body {masterPassword, codeHash,
                                                     kdfSalt, wrappedKey, wrappedIv}
  DELETE /recovery-key          (auth + CSRF)  -> remove config
  POST   /recovery-key/redeem   (NO auth)      body {username, code}
                                               -> session + wrappedKey + wrappedIv + kdfSalt
                                                  + user's current salt + kdfIterations

VerifyMasterPassword() helper handles both legacy 'pbkdf2' and
current 'pbkdf2-sha256' schemes consistently with PM.Handler.Auth.

Setup flow
==========
1. Settings → "Generate recovery code" button (asks master pw via reauth).
2. Client generates: 16-char code + fresh kdf_salt + exports the current
   AES key via crypto.subtle.exportKey('raw').
3. Client wraps the raw key under KEK=PBKDF2(code, kdf_salt, 600k)
   with a random 12B IV → base64.
4. POSTs to /recovery-key/setup. Server verifies master pw, INSERT-or-
   replaces the row (DELETE+INSERT, no UPSERT — same pattern as the
   lockout table since FireDAC's UPSERT support is patchy).
5. Confirm modal shows the plaintext code in a monospace, user-select-all
   panel. The modal is forcing: "I saved it" button is the only way out.
   Modal is the only place the code ever appears — server never sees it.

Redeem flow (forgot master pw)
==============================
1. Auth screen → "Forgot master password? Use a recovery code" link.
2. promptDialog: username, then code (masked input).
3. POST /recovery-key/redeem. Server hashes the typed code, joins with
   users by username, ConstantTimeEquals against stored hash. On match:
     - deletes the recovery_keys row (single-use)
     - issues a fresh session token + CSRF
     - returns: { token, csrfToken, salt, kdfIterations, kdfSalt,
                  wrappedKey, wrappedIv, userId }
4. Client unwraps the AES key with PBKDF2(code, kdfSalt, 600k) → raw bytes
   → importKey('raw') back into a CryptoKey.
5. State is reconstituted from the new session, persistCryptoKey, enterApp.
6. Client immediately opens the Change-master-password modal — the
   recovery code is consumed and the account needs a fresh master pw
   AND a fresh recovery code (the user generates a new one from Settings).

Backward compat
===============
Recovery is opt-in. Existing users see "No recovery key set" in Settings
until they generate one. No migration needed — the table is created via
CREATE TABLE IF NOT EXISTS at server startup, FK cascade on user delete.

Minor UI additions
==================
 - .btn-link CSS class for the auth-screen "Forgot master password?" link.
 - Recovery-status label in Settings refreshed on every openSettings()
   via GET /recovery-key/status.

delphi-backend/PMServer.dpr

@@ -19,7 +19,8 @@ uses
   PM.Handler.Auth in 'Handlers\PM.Handler.Auth.pas',
   PM.Handler.Folders in 'Handlers\PM.Handler.Folders.pas',
   PM.Handler.Entries in 'Handlers\PM.Handler.Entries.pas',
-  PM.Handler.Passkey in 'Handlers\PM.Handler.Passkey.pas';
+  PM.Handler.Passkey in 'Handlers\PM.Handler.Passkey.pas',
+  PM.Handler.Recovery in 'Handlers\PM.Handler.Recovery.pas';
 
 {$R *.res}
 {$R assets\assets.res}