fix(desktop): prevent multiple app instances with single-instance plugin

[usvimal]

Summary

• Adds tauri-plugin-single-instance to prevent multiple instances of OpenCode Desktop from running simultaneously
• When a second instance is launched, it focuses the existing window instead of spawning a new one

Problem

When launching OpenCode Desktop multiple times (intentionally or accidentally), the app spawns multiple instances. Each instance:

1. Calls get_sidecar_port() which binds to a random free port (since OPENCODE_PORT is typically not set)
2. Checks is_server_running(port) which returns false for the new random port
3. Spawns a new opencode-cli serve --port=XXXXX process

This leads to:

• Multiple windows opening unexpectedly
• Multiple background opencode-cli serve processes consuming resources
• Port exhaustion in extreme cases
• User confusion

Solution

Uses Tauri's official tauri-plugin-single-instance plugin:

1. On app launch, the plugin acquires a system-wide lock
2. If another instance already holds the lock, the new instance sends its arguments to the existing instance and exits
3. The existing instance receives a callback and focuses its window

This is the standard pattern recommended by Tauri for single-instance apps.

Changes

• Cargo.toml: Added tauri-plugin-single-instance = "2" dependency
• lib.rs: Added single-instance plugin initialization with window focus callback

Testing

• Launch app → works normally
• Launch app again → existing window focuses, no new instance spawned
• Close app → lock released, can launch fresh instance

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Duplicate Check Results

✅ No duplicates found

The PR you're checking appears to be unique. The only matching result across all searches is the PR itself (#6922).

The one tangentially related PR that appeared in the results:

• Add Intent abstraction for two-way server (and plugin) → client UI interactions #6549: "Add Intent abstraction for two-way server (and plugin) → client UI interactions" - This is unrelated to the single-instance functionality.

Conclusion: This PR is safe to proceed. There are no duplicate or conflicting PRs addressing the same single-instance plugin feature for preventing multiple app instances.