Support URL-encoded data-URL favicons

[evnchn]

Motivation

Developer passes favicon='data:image/svg+xml,%3Csvg%20.../%3E' (a valid, URL-encoded data URL, the same form NiceGUI generates internally) to ui.run(). is_file() returns False (data: guard), so startup wires get_favicon_response; a request to /favicon.ico calls _data_url_to_bytes -> b64decode drops the '%','<' etc. and returns corrupt bytes served as image/svg+xml. A value like 'data:foo' (no comma) instead raises ValueError -> HTTP 500. Fix: detect the ';base64' marker and branch to urllib.parse.unquote for non-base64 data URLs; guard the missing-comma case.

Minimal reproduction (nicegui/favicon.py:107):

from nicegui import core, favicon

svg = '<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16"><circle cx="8" cy="8" r="7"/></svg>'
data_url = favicon._svg_to_data_url(svg)
assert data_url.startswith('data:') and ';base64' not in data_url

core.app.config.favicon = data_url

try:
    favicon.get_favicon_response()
except Exception as e:
    print(f'BUG REPRODUCED: serving a NiceGUI-generated data-URL favicon raised '
          f'{type(e).__name__}: {e}')
else:
    raise SystemExit('BUG FIXED: data-URL favicon served without error')

→ BUG REPRODUCED: serving a NiceGUI-generated data-URL favicon raised Error: Incorrect padding

Implementation

Fixed _data_url_to_bytes to branch on ;base64 marker (base64 path byte-identical; percent-encoded uses urllib.parse.unquote) and partition() to drop the comma-less ValueError.

Verification

• fail-first test tests/test_favicon.
• Local gates: pre-commit · mypy ./nicegui · pylint 10.00/10 · pytest (touched) — all green.

Progress

• The PR title is a short phrase starting with a verb.
• The implementation is complete.
• This PR does not address a security issue.
• Pytests have been added/updated.
• Documentation is not necessary.
• No breaking changes to the public API.