Phase 3b red: login flow tests with SRP mock server

Adds fast-srp-hap (the same SRP library Ente's web client uses, pinned
to 2.0.4) as a runtime dependency.

Tests build a full mock Ente server using fast-srp-hap's SrpServer to
exercise real SRP-6a math end-to-end. The mock handles:
  GET /users/srp/attributes
  POST /users/srp/create-session
  POST /users/srp/verify-session
  POST /users/two-factor/verify
  POST /users/ott
  POST /users/verify-email

7 tests covering:
  * SRP login completing successfully
  * SRP login requiring TOTP (returns { kind: 'totp' })
  * Wrong password (SRP M1 fails server-side checkM1)
  * Email MFA fallback (returns { kind: 'emailOTP' })
  * submitTOTP
  * requestEmailOTP + submitEmailOTP

yarn.lock

@@ -879,6 +879,11 @@ fast-levenshtein@^2.0.6:
   resolved "https://registry.yarnpkg.com/fast-levenshtein/-/fast-levenshtein-2.0.6.tgz#3d8a5c66883a16a30ca8643e851f19baa7797917"
   integrity sha512-DCXu6Ifhqcks7TZKY3Hxp3y6qphY5SJZmrWMDrKcERSOXWQdMhU9Ig/PYrzyw/ul9jOIyh0N4M0tbC5hodg8dw==
 
+fast-srp-hap@^2.0.4:
+  version "2.0.4"
+  resolved "https://registry.yarnpkg.com/fast-srp-hap/-/fast-srp-hap-2.0.4.tgz#9db296e21a5143951310f99e5a74290106467811"
+  integrity sha512-lHRYYaaIbMrhZtsdGTwPN82UbqD9Bv8QfOlKs+Dz6YRnByZifOh93EYmf2iEWFtkOEIqR2IK8cFD0UN5wLIWBQ==
+
 fastq@^1.6.0:
   version "1.20.1"
   resolved "https://registry.yarnpkg.com/fastq/-/fastq-1.20.1.tgz#ca750a10dc925bc8b18839fd203e3ef4b3ced675"