feat(auth): #65: keep master key in memguard enclave via XOR split

[imdeepmind]

Summary

Keeps the plaintext master key out of heap memory and the serialized Session struct by moving it into a memguard enclave (locked, unswappable, encrypted RAM) split via XOR.

Changes

• XOR split (internal/shared/crypto/encryption.go): SplitKey, CombineKey, and CombineKeyToBuffer split a 32-byte master key into two equal-length pieces (A ⊕ B = Key). Neither piece alone reveals the key.
• Piece A lives in a memguard.NewEnclave on the in-memory Session (unexported field), never serialized to the frontend.
• Piece B lives in the OS Keyring (pieceb_<username> entries) via the auth repository, deleted on logout.
• Session.MasterKey() removed in favor of Service.WithMasterKey(fn) — a Wails-bound method that reconstructs the key in a locked memguard buffer, runs fn, and destroys it immediately. errors package sentinels/AsInternalServerError used for all failure paths.
• Consumers migrated to WithMasterKey: settings (encrypt/decrypt settings) and upload (WrapDEK/UnwrapDEK in the processor).
• Regenerated frontend/wailsjs bindings for the new WithMasterKey method.

Verification

• go build ./..., go vet ./... pass
• golangci-lint run — 0 issues
• Frontend format:check + lint pass
• gofmt clean