fix(devices): correct desktop icon relative path for Desktop install location

[nu-jliu]

Summary

The .desktop launcher files install to /home/i2rt/Desktop while the repo is cloned to /home/i2rt/i2rt. The Icon= paths were relative to the file's in-repo location (devices/), so they broke once the files were copied to the Desktop. This corrects the relative Icon= paths to resolve from /home/i2rt/Desktop to the icon assets in the repo.

Changes

• devices/FlowBase.desktop: update Icon= to ../i2rt/i2rt/flow_base/assets/flow_base.png so it resolves from /home/i2rt/Desktop to /home/i2rt/i2rt/i2rt/flow_base/assets/flow_base.png
• devices/FlowBaseGamepad.desktop: same Icon= path correction (flow_base.png)
• devices/LinearRailVehicle.desktop: update Icon= to ../i2rt/i2rt/flow_base/assets/linear_rail.png for the new install location
• devices/LinearRailVehicleGamepad.desktop: same Icon= path correction (linear_rail.png)

Test Plan

• Clone the repo to /home/i2rt/i2rt and copy the four files from devices/ into /home/i2rt/Desktop.
• Confirm /home/i2rt/i2rt/i2rt/flow_base/assets/flow_base.png and linear_rail.png exist.
• From /home/i2rt/Desktop, run readlink -f ../i2rt/i2rt/flow_base/assets/flow_base.png and verify it resolves to the file above.
• Open the file manager / desktop and confirm each launcher shows its icon (FlowBase + Gamepad → flow_base.png; LinearRailVehicle + Gamepad → linear_rail.png).
• Edge case: if a desktop environment ignores relative Icon= paths, note that absolute paths may be required (per-spec, Icon= expects an absolute path or a bare theme name).

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