Improve terminal symbol font fallbacks

[askac]

Summary

• add Cascadia Mono to the Windows terminal fallback chain so standard symbols missing from Consolas render in a monospaced font
• migrate the previous Windows default font chain without overwriting custom font settings
• add an optional Powerline symbol fallback in Appearance for prompt glyphs such as U+E0A0
• vendor the 2,264-byte official PowerlineSymbols font with its MIT license, upstream commit, and SHA-256 provenance
• apply the setting immediately to visible terminals and agent mirrors while keeping it off by default

Validation

• tools/.venv_wsl/bin/python scripts/run_smoke_tests.py
• tools/.venv_wsl/bin/python -m py_compile tests/agent_browser_smoke.py
• tools/.venv_wsl/bin/python tests/agent_browser_smoke.py
• verified PowerlineSymbols.otf SHA-256 and font coverage for U+E0A0-E0A2 and U+E0B0-E0B3
• git diff --check

Compatibility

• existing custom font settings remain unchanged
• macOS default text fonts remain unchanged
• the bundled Powerline font is loaded only when the optional fallback is enabled