feat: intent-led homepage UX redesign

[aaronbrezel]

Summary

• Replaces feature-named buttons and arbitrary section headers with a flat, intent-led list under an "I want to..." sentence stem
• Buttons now use two-line copy: a direct action title + a journalist-context subtitle (e.g. "Summarize a Drive folder" / "For FOIA drops, court filings, doc sets")
• "Summarize a Drive folder" navigates directly to the recipe panel, removing the intermediate recipes-list submenu hop
• Adds .home-prompt CSS class for the sentence stem

Test Plan

• All 491 tests pass (npm test)
• Open sidebar in Sheets — verify "I want to..." prompt renders above buttons
• Click each button — verify correct panel opens
• Click "Summarize a Drive folder" — verify recipe panel opens directly (no recipes-list screen)
• Click "Pull a random sample" — verify job strip shows label "Pull a random sample" (not the full nested text)