fix(app-template): write extension example paths with TOML-safe forward slashes

[ndonkoHenri]

flet create --template extension generates an example app whose pyproject.toml cannot be parsed on Windows, so the very first thing the user-extensions guide tells you to do — build or run the generated example — fails before it starts.

The two entries that point the example back at the extension package interpolated the host's os.sep into TOML basic strings:

[tool.flet.dev_packages]
flet-wvr = "..{{cookiecutter.sep}}.."           # -> "..\.."   on Windows

[tool.uv.sources]
flet-wvr = { path = "..{{cookiecutter.sep}}..{{cookiecutter.sep}}", editable = true }  # -> "..\..\"

create.py passes "sep": os.sep, so on Windows the first renders "..\.." — \. is not a valid TOML escape — and the second renders "..\..\", where the trailing \" escapes the closing quote and leaves the string unterminated. That second one is the "unbalanced quotes" error reporters hit; tomllib reports it as Unescaped '\' in a string. POSIX was unaffected, since sep is / there.

Both paths are now written with plain forward slashes, which need no escaping in TOML and which pathlib (how [tool.flet.dev_packages] is resolved) and uv accept on Windows exactly as elsewhere. {{cookiecutter.sep}} no longer appears anywhere under sdk/python/templates/app/; the build template's use of it for Kotlin source directories is untouched, as that one really is a filesystem path.

This was fixed once, in August 2025, but only on the archived flet-app-templates 0.28.3 branch — it was never forward-ported when the templates moved into the monorepo.

Refs #5507. That issue reports two separate problems; this fixes the template half only. The packaging hang tracked there is unrelated to this change and stays open.

Test code

# Renders the extension template as Windows would (os.sep == "\") and parses
# every generated pyproject.toml. Fails on main, passes on this branch.
import pathlib, tempfile, tomllib
from cookiecutter.main import cookiecutter

out = tempfile.mkdtemp()
cookiecutter(
    template="sdk/python/templates/app",
    directory="extension",
    output_dir=out,
    no_input=True,
    extra_context={
        "template_name": "extension", "flet_version": "1.0.0", "sep": "\\",
        "platform": "windows", "out_dir": "flet_wvr", "project_name": "flet-wvr",
    },
)
for p in pathlib.Path(out, "flet_wvr").rglob("pyproject.toml"):
    tomllib.loads(p.read_text())   # main: tomllib.TOMLDecodeError: Unescaped '\' in a string
    print("OK", p)

Result on this branch, under both sep="\\" and sep="/":

OK flet_wvr/pyproject.toml
OK flet_wvr/examples/flet_wvr_example/pyproject.toml
   [tool.flet.dev_packages] = {'flet-wvr': '../..'}
   [tool.uv.sources]        = {'flet-wvr': {'path': '../../', 'editable': True}}

Summary by Sourcery

Ensure generated extension examples remain valid and buildable across platforms.

Bug Fixes:

• Fix extension template projects generating Windows-invalid TOML paths by using forward-slash package references in generated example applications.

Enhancements:

• Correct the macOS architecture help text to use the x86_64 architecture name.

[sourcery-ai]

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