✨ feat(tui): prioritize conversation and ask visibility

[zrr1999]

Summary

• Prioritize the conversation surface across compact, standard, and wide terminal viewports with a dedicated native TUI layout model.
• Keep pending Ask interactions visible through the native conversation workbench, including lifecycle transitions, fallback handling, bounded replay, and timeout presentation.
• Preserve selected theme behavior while adding a deterministic fallback for unknown or unavailable theme values.

Behavior

• Layout tests cover viewport thresholds and verify that conversation content remains primary while secondary surfaces collapse predictably.
• Ask coverage exercises active, resolved, fallback, replay/deduplication, and timeout states in the native harness.
• Theme coverage verifies selected themes and fallback resolution.
• Protocol, session, Ask package, Cockpit, daemon, and repro code are unchanged.

Scope

Exactly seven paths are changed:

• apps/spark-tui/src/host/theme.ts
• apps/spark-tui/src/native-tui/app.ts
• apps/spark-tui/src/native-tui/layout.ts
• apps/spark-tui/src/native-tui/theme-helpers.ts
• test/spark-native-tui-harness.test.ts
• test/spark-native-tui-layout.test.ts
• test/spark-theme.test.ts

Verification

• Focused root suite: 10 files, 170 tests passed.
• pnpm run check:static passed.
• pnpm run test:process:source passed: 1 file, 1 test.
• pnpm exec vp check --no-fmt --no-lint apps/spark-tui passed: 69 files, 0 type errors.
• git diff --check origin/main...HEAD passed.

Warnings

• Static checks report existing repository warnings: 9 lint warnings overall and 3 generated i18n runtime type warnings under packages/spark-i18n/src/paraglide/runtime.js; there are 0 errors and this PR does not modify those files.