feat(tui): add opt-in transparent backgrounds

[TheGreatAxios]

Summary

• make transparent backgrounds an explicit opt-in for root, transcript, landing, welcome, provider, and pending-queue canvas surfaces while keeping prompts, overlays, and selected controls opaque
• select the startup palette before first-run welcome and provider surfaces using explicit setting, COLORFGBG, synchronous macOS appearance, then dark fallback
• render active provider and selected pending-queue rows on the theme's opaque ground while inactive rows remain canvas-transparent
• document that transparent-canvas readability depends on matching the selected or detected theme to the host background
• keep this PR stacked on feat(tui): add automatic light theme selection #1217 (cl-8993-light-palette) at 72253d8b62fe9ed3a95b84b0ee84e87425ee9414

Verification

• bun test src/tui/theme-transparent.test.ts src/tui/shell.test.ts passes 61 tests across both files
• headless dark/light transparent renders verify welcome/provider/pending canvas alpha 0, selected provider and pending rows alpha 255, body contrast at least 4.5, inactive rows alpha 0, and pending selection transfer
• bun run check passes formatting, lint, typecheck, dead-export checks, build, and the guarded randomized full test suite
• git merge-base HEAD cl-8993-light-palette resolves to 72253d8b62fe9ed3a95b84b0ee84e87425ee9414

Fixes CL-9024

[linear-code]

CL-9024