fix(generator): fall back to the login link on cp1252 consoles

[MohammedAlkindi]

Fixes #150.

On Windows, redirected stdout falls back to the ANSI code page (cp1252). The half-block characters from print_ascii cannot be encoded, so _render_qr dies with UnicodeEncodeError before any login path is shown. Reproduced on Windows 11, Python 3.13, piped stdout.

Two changes in session_string_generator.py:

• _render_qr now checks whether stdout can encode the rendered art and prints the tg:// login link instead when it cannot. Same fallback idea qsardor proposed in the issue body; I test encodability up front instead of catching, because with hardened stdio (below) the print no longer raises, it would just draw an unscannable grid of ?.
• main() reconfigures stdout/stderr with errors="replace". As tonydzi noted, the QR art is only the first member of this failure class: the account label and error text also reach stdout and can carry non-latin characters. This stops any of them from aborting a login, and it helps users who cannot set PYTHONUTF8=1 in their launcher.

Three regression tests added. The fallback test fails on main with the exact error from the issue. black, flake8 and the whitespace hooks pass on the changed files; the docker compose hook needs a local .env and passes once one exists.

Verification on Windows 11, Python 3.13

Before, piped stdout under cp1252:

File "session_string_generator.py", line 80, in _render_qr
    print(f.getvalue())
UnicodeEncodeError: 'charmap' codec can't encode characters in position 0-26

After, same environment:

This console (cp1252) cannot draw the QR code.
Open this link on a device where you're logged in instead:
  tg://login?token=...
Expires at: 11:29:03
Waiting for you to scan...

pytest tests/test_session_string_generator.py: 15 passed.
Full suite: 276 passed, 8 failed identically on main and on this branch
(POSIX permission and pool-lock tests that do not hold on Windows).