feat(smime): client-side certificate enrolment — web S/MIME now fully functional

New enroll.js: generates an RSA-2048 keypair with WebCrypto (extractable
only long enough to export to PKCS#8), builds and signs a real CSR with
pkijs (same per-call-engine convention as smime-sign.js/smime-verify.js —
nativeEngine() passed explicitly, no global pkijs.setEngine call), POSTs
it to the already-existing /api/smime/enroll (same-origin fetch — the
plugin's privileged tier gets allow-same-origin, cookies included by
default), and packages the result into a key record using the EXACT same
encrypted-at-rest convention as a PKCS#12 import (AES-GCM/PBKDF2 600k,
exported from pkcs12.js) so every downstream sign/encrypt/decrypt/verify
path is identical regardless of how the key arrived.

New "Get a certificate" button in the settings-section UI, next to
"Import key" — prompts for a storage passphrase, calls enroll(), saves
the key record, and refreshes the list. No changes needed to the CA route
or the CA provider — both were already real and already tested.

Live end-to-end verified (not just unit-level): logged in via the real
dev-mode session flow, clicked through the actual plugin UI, got back a
real certificate (RSA-2048, correct validity window, real fingerprint) for
dev@localhost, then unlocked it with the same passphrase — the encrypted
private key round-trips correctly through the identical code path a
PKCS#12 import would use.

Also fixes a real bug hit during that verification: SESSION_SECRET must be
>= 32 chars (lib/auth/crypto.ts), but .env.dev.example's own documented
placeholder was 29 - failing "Failed to store Stalwart auth context" on
every feature needing the real session-cookie flow (this enrolment route,
offline sync, AI server class). Anyone following the setup doc verbatim
would have hit this. Padded the placeholder to 37 chars.

.env.dev.example

@@ -35,7 +35,7 @@ APP_NAME=Bulwark Webmail (Dev)
 # Session & Settings Sync (optional for dev)
 # =============================================================================
 
-SESSION_SECRET=dev-secret-not-for-production
+SESSION_SECRET=dev-secret-not-for-production-32chars
 SETTINGS_SYNC_ENABLED=true
 
 # =============================================================================