Fix polygon footprint preview (#133)

[sfoale]

Fixes #133

• Parse polygon preview input with parse_multi_polygon instead of json.loads so the [(x,y)...]#[...] format renders.
• Surface back end errors in the front end instead of silently failing to plot.
• Show a login-specific message for 401/403 on instrument submit from the website.
• Add tests.

[Copilot]

Pull request overview

This PR fixes polygon footprint preview rendering in the UI by aligning polygon parsing with the backend’s footprint parser, improving error surfacing to the frontend, and adding FastAPI tests for polygon preview behavior.

Changes:

• Update /ajax_preview_footprint to parse polygon input via parse_multi_polygon (supporting [(x,y)...]#[...] and newline-separated formats) and return parse errors to the client.
• Improve Plotly polygon rendering semantics (mode="lines", fill="toself") for footprint previews.
• Frontend: surface backend { error } preview responses as exceptions, and show a login-specific message on 401/403 instrument submit failures; add UI preview tests for polygon cases.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tests/fastapi/test_ui.py	Adds test coverage for single/multi/invalid polygon preview responses.
server/routes/ui/preview_footprint.py	Switches polygon preview parsing to parse_multi_polygon, returns parsing errors, and adjusts Plotly trace fill behavior.
frontend/src/lib/services/instrumentService.ts	Improves error handling for preview responses and adds clearer 401/403 messaging for instrument submission.

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[swyatt7]

Moira and I tested this. Works as expected :)

thanks

[moira-andrews]

Tested and was able to preview footprint successfully