feat: add dynamic theme changes

[loeck]

Summary

• Add ghostty_terminal_set_colors WASM export that updates terminal colors at runtime and forces a full redraw
• Wire up terminal.options.theme = newTheme to merge partial themes, update the renderer, and sync WASM terminal colors
• Existing content re-renders instantly — cells with ANSI palette/default colors pick up the new theme; explicit RGB cells remain unchanged
• Support partial theme updates that accumulate (e.g. setting only background preserves all other colors)

Usage

// Full theme swap
terminal.options.theme = { background: '#000', foreground: '#fff', red: '#ff0000' };

// Partial update — only changes background, keeps everything else
terminal.options.theme = { background: '#1a1a2e' };

// Reset to defaults
terminal.options.theme = {};

Changes

File	Change
patches/ghostty-wasm-api.patch	New setColors() Zig function, force_full_redraw flag on TerminalWrapper, render state reset on color change
ghostty-vt.wasm	Rebuilt with ghostty_terminal_set_colors export
lib/types.ts	Added ghostty_terminal_set_colors to WASM exports interface
lib/ghostty.ts	Added setColors() method to GhosttyTerminal class
lib/terminal.ts	handleOptionChange('theme') now merges, updates renderer + WASM; added currentTheme accumulator and buildThemeColorsConfig() helper
lib/terminal.test.ts	12 new tests covering full/partial/reset/accumulation/ANSI re-resolution/RGB unchanged/redraw trigger

Test plan

• Full theme change updates renderer and WASM colors
• Partial theme updates preserve previous customizations
• Successive partial updates accumulate correctly
• Reset to null/undefined/{} restores defaults
• Theme set before open() is applied correctly
• ANSI palette color cells re-resolve after theme change
• Explicit RGB color cells remain unchanged
• Theme change triggers full redraw (needsFullRedraw() returns true)
• Invalid color values do not crash
• Default fg/bg cells update after theme change
• bun run typecheck && bun test pass (343 tests, 0 failures)

Ported from coder/ghostty-web#144