chore: simplify mail bridge life cycle

[AlexisMora]

What is Changed / Added

Replaces the previous Mail Bridge manager/state lifecycle with focused, functional services.
Desktop now separates:

• Process concerns: resolve the Bridge executable, create the per-account state directory, spawn the child process, and capture sanitized startup stderr.
• Startup concerns: wait for the private control connection and Bridge ready response, with a 30-second timeout and child exit/error handling.
• Resource cleanup: destroy the control socket, close the private control server, and terminate the child process.
• Lifecycle ownership: expose start, stop, current status, and status subscriptions from one Desktop service.

The new lifecycle service also coalesces concurrent start requests: a second activation waits for the first startup operation rather than starting another Bridge process.

Behaviour

• Starts the Bridge through a private named-pipe control endpoint.
• Sends start_session only after the Bridge connects.
• Returns actual local IMAP/SMTP settings after ready.
• Surfaces startup failures.
• Stops resources on startup failures, logout, app quit, and explicit turn-off.
• Handles child-process and control-channel failures as lifecycle errors.

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[github-actions]

This PR changes 737 lines.

If it contains multiple concerns, consider splitting it. Large PRs are fine when the changes are mechanical, intentionally grouped, or belong together as part of the same feature.

Please make sure the PR description gives reviewers enough context about what matters most to review.