fix(webui): correct launch flags and add Python version compatibility check (fixes #108)

[BerryUIKI]

Summary

Correct WebUI launch flags to use proper boolean --listen and separate --server-name flags, remove --nowebui to enable native UI access, and add Python version compatibility checks.

Changes

• Correct --listen flag: Use boolean --listen with separate --server-name 127.0.0.1 instead of --listen 127.0.0.1
• Enable native WebUI: Remove --nowebui flag to allow both API and native WebUI access
• Python version check: Add compatibility check warning for non-3.10.x Python versions in isolated runtime
• Preserve API access: Keep --api flag for REST API access
• Preserve model paths: Keep --ckpt-dir, --lora-dir, --vae-dir flags for model discovery
• Comprehensive tests: Verify correct flag usage, native UI enablement, and version compatibility

Problem Solved (Issue #108)

Before:

• --listen 127.0.0.1 incorrectly treated 127.0.0.1 as a boolean flag value
• --nowebui disabled the native WebUI interface
• No Python version compatibility check despite WebUI expecting Python 3.10.6

After:

• Correct loopback binding: --listen (boolean) + --server-name 127.0.0.1
• Both API and native WebUI are accessible
• Runtime logs warning when Python version differs from WebUI's expected 3.10.x

Acceptance Criteria (Issue #108)

• Use correct loopback binding flags (--listen boolean + --server-name)
• Both API health and advertised native WebUI are accessible
• Python version compatibility check logs actionable warnings
• Unsupported prerequisites provide guidance for resolution

Testing

All tests in backend/tests/test_webui_launch_flags.py verify:

• Correct flag structure: --listen followed by --server-name 127.0.0.1
• --nowebui flag is NOT present
• --api flag is preserved for API access
• Python version compatibility check warns for non-3.10.x versions
• Model directory flags are preserved

Related Issues

Fixes #108

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