fix(app): preserve model selection across draft remounts

[opencode-agent]

Summary

• scope transient model selection to each draft tab
• persist draft model state across provider and route remounts
• preserve the promotion handoff behavior merged in fix(desktop): preserve selected model during session promotion #34466
• add regression coverage for independent draft selections

This isolates the remaining unmerged part of #33858 on current dev.

Testing

• bun test --preload ./happydom.ts ./src/context/model-selection-state.test.ts ./src/components/prompt-input/submit.test.ts (6 passed)
• bun typecheck
• bun run test:browser (30 passed)
• production first-navigation benchmark, 10 serial runs per ref: 0 blank and 0 unknown samples before and after; median stable destination 71.3ms before / 71.9ms after
• Playwright real-app flow: select Sonnet in draft A, select Opus in draft B, return to draft A; before resets to Opus, after restores Sonnet

bun run test:unit has the same existing i18n parity failure on origin/dev: Arabic is missing three session.header.reveal.* keys (571 other tests pass).

[NightmareZero]

Why was it closed without being merged first?