fix: harden Linux pairing credential storage

[enaboapps]

Closes #720

Summary

Wrap Linux Secret Service-backed token storage with serialized operations, nonempty save/read-back verification and fixed actionable errors. Preserve keyring identity, state schema, missing-entry semantics and Windows/macOS behavior. Failed read-back never deletes credentials. Add four fake-store/model recovery tests and documentation. Linux pairing/input remain disabled.

Validation

• Frontend lint, 114 tests, 5 updater tests and build passed locally.
• Rust formatting and git diff --check passed.
• Local native Clippy/tests blocked by absent GLib/GDK development packages; all native CI jobs passed on Linux, Windows and macOS.
• CI 34618899057 and CodeQL 34618899126: all green.
• Independent latest-head review db48185: no actionable findings.
• Tests use fake stores only, never real credentials or input.

No claim of physical Secret Service durability, transactional replacement or subscriber isolation. Production integration still requires bounded off-main-thread operations and hardware qualification. Independent of #719, targets linux-support; not merged or released.

[enaboapps]

Independent latest-head review of db48185: no actionable findings. Reviewer confirmed Linux-only production selection, stable identity, serialization, read-back verification, sanitized errors and non-destructive failure behavior. Formatting passed; native compilation/tests still depend on CI because this host lacks development libraries.

[greptile-apps]

Greptile Summary

This change adds a Linux-specific credential-store wrapper that serializes access, rejects empty values, verifies writes through read-back, and returns a safe remediation message instead of backend details. It also documents the operating constraints and remaining readiness work for Linux credential storage.

T-Rex validation blocked

The focused credential recovery check could not compile because the environment lacks the GDK 3 development metadata file (gdk-3.0.pc). No defect was established.

Confidence Score: 5/5

No confirmed issue prevents merging this change.

No confirmed findings were produced. The focused runtime check was blocked before its test binary could be built because a required system development dependency is unavailable.

Files Needing Attention: No source file requires follow-up from this review; Linux credential recovery should be exercised in an environment with the required native desktop libraries before Linux pairing is enabled.

T-Rex Logs

What T-Rex did

• T-Rex attempted the focused credential recovery test to verify preservation of pairing metadata during credential-store failure and restoration after recovery.
• The test could not progress to compilation because pkg-config could not locate gdk-3.0.pc, so the test binary was not built and the runtime path was not executed.
• Cargo output confirmed the environment blocker: pkg-config failed to find gdk-3.0.pc with PKG_CONFIG_PATH unset, and no source code changes were made.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
src-tauri/src/storage.rs	Adds a Linux credential-store wrapper with serialized operations, verified writes, and safe error handling.
docs/linux-credentials.md	Documents Linux credential-storage behavior, limitations, validation coverage, and remaining enablement gates.

_{Reviews (1): Last reviewed commit: "fix: harden Linux pairing credential sto..." | Re-trigger Greptile}