feat: establish Linux development runtime and native CI

[enaboapps]

Linux currently cannot compile because its runtime dispatch and cursor-overlay implementation are missing. This provides a development-only Linux runtime that reports Bluetooth and input unavailable, rejects pairing, clears session state, and opens no input device or overlay windows.

The Linux main window stays visible even with --start-hidden, and closing it follows the existing confirmation flow to quit, so an absent tray host cannot strand the app. Ubuntu 24.04 joins native CI for format, Clippy, tests and a development build. Shared code remains covered by fake-input tests.

Stack: #711 (plan) → this PR → #715 (Linux setup/status UX). The base is the planning branch as requested for stacked implementation. BlueZ and actual input support follow separately. Merge in stack order only when explicitly authorized.

Closes #712.

Validation: local formatting, whitespace and frontend checks passed. Independent review of latest head 290712dd8734aeab0232742866a1281f00153447 found no actionable issues. All latest-head CI checks passed: Ubuntu, Windows and macOS native checks/builds; frontend; dependency audit; CodeQL; and automated review. Native/frontend CI run.

The connection-history test uses an explicit in-memory pairing-token store so it does not require a desktop keyring. Native credential-store tests retain their original adapters and production storage is unchanged.

Linux hardware and graphical runtime behavior have not been qualified. Local native checks were blocked by unavailable GTK/WebKit development packages; native validation runs in CI. No Linux release artifacts or functional Bluetooth/input support are provided by this PR.

[greptile-apps]

Greptile Summary

This change establishes a development-only Linux runtime that reports Bluetooth, input, dwell, and overlay capabilities as unavailable rather than attempting unsupported operations. It keeps the main window recoverable without tray integration, adds Linux-native CI checks, and uses in-memory credentials for persistence tests that do not require a desktop keyring.

The Linux start-hidden and close behavior was exercised successfully: start-hidden is disabled when no tray is available, closing requests quit, and the built application remained running under an Xvfb and D-Bus session. The unavailable-runtime reset path, native formatting, strict linting, complete Rust test suite, and debug no-bundle build also completed successfully. No defects were found.

Confidence Score: 5/5

Safe to merge based on the exercised Linux lifecycle, fail-closed unavailable-feature behavior, native quality checks, test suite, build, and graphical startup probe.

No publishable findings remain after execution verified the principal Linux lifecycle and unavailable-feature failure paths. The Rust formatter, strict Clippy checks, complete native test suite, and debug application build all succeeded.

Files Needing Attention: No files need corrective attention. Future Linux feature work should continue to validate real Bluetooth hardware, desktop input, and overlay behavior before enabling those capabilities.

T-Rex Logs

What T-Rex did

• The focused Linux lifecycle tests passed, including the --start-hidden disabled check and the close-quit behavior, and the unavailable-runtime reset test passed with the active state cleared and the credential preserved.
• The Rust validation and native build checks completed: formatting and Clippy passed, 211 library tests and 7 configuration tests succeeded, and the no-bundle Tauri debug run produced the Linux executable.
• The built Linux application was started under a direct Xvfb display and D-Bus session with --start-hidden, and it remained running through the timeout without crashing or entering an unrecoverable hidden state.
• A consolidated review of all artifacts from the Linux lifecycle, native validation, and start-hidden runtime runs was performed to verify recoverability and build outputs.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "test: retain native storage coverage wit..." | Re-trigger Greptile}