feat(go-internal): #70: consolidate key management, move dialog to shared, fold recovery into auth

[imdeepmind]

Fixes #70

• Consolidate key-management code: move the dbconfig/masterkey features into the auth feature and lift crypto primitives (DualEncrypt/DualDecrypt, Wipe, password hashing, recovery-key helpers) into internal/shared/crypto; scrub plaintext key material from memory after use.
• Move internal/platform/dialog to internal/shared/dialog (stateless utility, no client dependency; used directly by multiple features).
• Fold the recovery feature into auth (auth.Service.SaveRecoveryKey via shared/dialog), dropping the separate recovery service, bindings, and AGENTS.md entry.

Verified with go build, go vet, frontend lint + type-check, and a full wails build.