fix(terminal): new terminals respect selected color scheme

[gregpriday]

Summary

• New terminals (including pre-warmed pool instances) were always initialized with the default CSS-variable theme, ignoring any color scheme selected in Appearance settings
• Both TerminalRegistryController.prewarm() and the XtermAdapter terminalOptions memo called getXtermOptions() without a theme argument, falling back to getTerminalThemeFromCSS()
• The fix adds a selectEffectiveTheme selector to terminalColorSchemeStore and passes the resolved theme at construction time in both code paths

Resolves #3335

Changes

• src/store/terminalColorSchemeStore.ts — adds selectEffectiveTheme selector and a getEffectiveTheme() snapshot helper for use outside React; deduplicates the theme computation that was previously inlined in two places
• src/controllers/TerminalRegistryController.ts — reads getEffectiveTheme() from the store snapshot when building xterm options for pre-warmed terminals
• src/components/Terminal/XtermAdapter.tsx — subscribes to selectEffectiveTheme in the terminalOptions memo so new instances created during React mount use the current scheme
• src/store/slices/terminalRegistry/index.ts — passes getEffectiveTheme() when creating terminal instances inline (non-pool path)

Testing

• npm run check passes (typecheck, lint, format all clean)
• Manually verified: spawning a new terminal after selecting a custom scheme opens with that scheme applied immediately
• Existing terminals continue to update live when the scheme changes via applyGlobalOptions in useTerminalConfig
• Default (CSS-variable) theme still applies correctly when no custom scheme is selected

[gregpriday]

Review status: Ready

Clean refactor of getEffectiveTheme() into computeEffectiveTheme() + selectEffectiveTheme selector. Correctly wired into XtermAdapter, TerminalRegistryController.prewarm(), and terminalRegistry slice.

Cross-PR note: This PR must merge before PR #3345 (match terminal scheme to app theme). Both modify terminalColorSchemeStore.ts and XtermAdapter.tsx. PR #3345 will need a rebase after this merges — the getEffectiveTheme() inline code it modifies has been extracted to computeEffectiveTheme().