Explain a too-large share instead of showing "server returned 413"

[erikdarlingdata]

Summary

• nginx on the PlanShare server now accepts shares up to 10 MB (changed on the server today, not in this PR). nginx had no client_max_body_size, so its 1 MB default applied, while the app accepts 10 MB. Shares from 1 to 10 MB failed with "Share failed: server returned 413". I added client_max_body_size 10m; to the /api/ location, ran nginx -t, and reloaded.
• This PR fixes what users see above 10 MB. nginx and Kestrel both still refuse those uploads. Neither sends a JSON error, so the web client showed "server returned 413". It now shows: "This plan is too large to share. The limit is 10 MB."
• The nginx setting lives only on the server. The comment on the Kestrel limit now records it, so the three places that state 10 MB change together.

Changes

• src/PlanViewer.Web/Services/PlanShareService.cs: a 413 without a JSON error gets the size message. When the server does send error text, that text still wins.
• server/PlanShare/Program.cs: comment only. It names the nginx setting and the web client's message.
• tests/PlanViewer.Core.Tests/PlanShareServiceTests.cs:
  • A new test covers both kinds of 413 reply: nginx's HTML page and Kestrel's empty body.
  • The existing server-text test gets one more case: a 413 that carries error text shows that text.

The server change

• The backup of the old site file is /root/nginx-backups/stats.erikdarling.com.20260929T193036Z.
• Before the change, a 2 MiB POST to /api/share got nginx's 413.
• After the change:
  • 2 MiB and 9 MiB reach the app. The app answers 400, because the test body is a JSON array, not a share, so nothing was stored.
  • 11 MiB still gets nginx's 413.
• nginx and planshare are both active.
• The deploy workflow never writes the nginx site file, so a deploy will not undo this.

Test Plan

• PlanShareServiceTests: 15 passed, 0 failed.
• CI runs the full suite on this PR.
• After the next release deploys the web viewer, a share over 10 MB shows the new message.

The Program.cs comment is under server/PlanShare/**, so the next release merge redeploys PlanShare with no change in behavior.

Generated with Claude Code

https://claude.ai/code/session_019tS6P95Dtzs4aeMpb1xqzX

[claude]

Reviewed: no findings. The 413 fallback runs only after the server's JSON error text is checked, so server text still wins. The tests cover the HTML (nginx) and empty-body (Kestrel) cases, and the size-limit comments point at all three places that hold the 10 MB limit. I didn't confirm that the nginx client_max_body_size 10m setting the new comment describes exists in the server config, since that config isn't in this repo.