fix(ui): bundle Windows caption icons

[Tryanks]

What changed

• render Windows minimize, maximize/restore, and close controls with the SVG icons already embedded by gpui-component-assets
• remove the Segoe Fluent Icons system-font and PUA glyph dependency
• add a test proving all four caption SVGs load through the tcode Assets facade

Why

PR #133 followed the Furray implementation and used Segoe Fluent Icons. That font ships with Windows 11 but is not included by default on Windows 10, where private-use glyphs cannot rely on normal font fallback. The existing GPUI component asset bundle already provides deterministic SVG resources for all four caption states.

User impact

Caption controls now render from assets embedded in the application binary on every supported Windows version, while native WindowControlArea hit testing, maximize/restore switching, hover states, and layout remain unchanged.

Validation

• cargo fmt --all -- --check
• cargo test -p tcode-ui window_caption — 6 passed
• cargo test -p tcode-ui assets — 1 passed and loaded all four SVGs
• cargo check -p tcode
• cargo check -p tcode --target x86_64-pc-windows-gnu