Add ui.status_code() to set HTTP status codes from page builders

[evnchn]

Motivation

Page builder functions currently have no way to control the HTTP status code of the response. This is needed for proper SEO (returning 404 for not-found pages) and for any scenario where a page needs to signal a non-200 status.

Split out from #5767 per review feedback — this is a public API addition that deserves focused review.

Implementation

• nicegui/functions/status_code.py (new): ui.status_code(code) sets context.client.status_code
• nicegui/client.py: Adds status_code field to Client, uses it in build_response() when set
• nicegui/ui.py: Exports status_code in __all__ and imports
• main.py: Uses ui.status_code(404) for unknown documentation pages
• tests/test_page.py: Tests for custom and default status codes

Open questions for review

• Should code be validated (e.g. must be in 100-599 range)?
• Should it return the previous code?
• Does it need documentation on the website?

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• If this PR addresses a security issue, it has been coordinated via the security advisory process.
• Pytests have been added (or are not necessary).
• Documentation has been added (or is not necessary).

🤖 Generated with Claude Code

[falkoschindler]

Quick clarification:

This is needed for proper SEO (returning 404 for not-found pages)

Undefined routes already return 404. Or am I missing something?

@ui.page('/')
def page():
    ui.label('Hello, world!')

$ curl -I http://127.0.0.1:1234/foo
HTTP/1.1 404 Not Found
...

[falkoschindler]

Ah! You're focusing on scenario 2:

---

The distinction is between two different scenarios:

1. No matching route (e.g., visiting /totally-random-url): NiceGUI/FastAPI already returns HTTP 404 — you're right about this.

2. Route matches but content doesn't exist (e.g., /documentation/{name} where name is nonsense): The {name} wildcard catches the request, the page builder runs, renders the "not found" label, and returns HTTP 200 — because from the framework's perspective, the page builder succeeded.