Fix header reveal flicker while scrolling

[daphnehanse11]

Summary

• stagger the sticky header background and compact brand/nav reveal so the compact title does not fade over the hero heading
• make the mobile benchmark header fully populated from the top, with country pills and section tabs fitting at phone widths
• make the paper page header scroll normally instead of staying sticky over the manuscript
• remove the Global tab and default visitors to US unless browser timezone/language indicates UK
• reorder the app flow to model rankings -> program breakdown -> scenarios -> failures -> methodology
• keep the default model-ranking path upfront and move scoring/reference/weighting controls into a collapsed Adjust ranking panel below the table
• move scenario explanation/audit coverage and exact prompt below the household output table
• move What the error reads show into the failures section
• make scenario prediction values visibly interactive and align the program legend with the table color thresholds
• rename the failure-mode callout from "Read this carefully" to "How to read these cards"
• remove the Hardest household segments section from failure modes
• add the teal PolicyEngine mark as the site favicon
• remove ACA premium tax credit from rendered website payloads, public paper/scoring pages, prompts, program displays, weights, and displayed scoring

Fixes #44.

Testing

• npm run lint
• npm run build
• rendered text check confirms the section order is model rankings -> program breakdown -> scenario explorer -> failures -> methodology
• rendered text check for /, /paper, and /metric-options.html found no ACA/Marketplace/premium tax credit terms
• Playwright screenshots: desktop benchmark flow, mobile benchmark flow, mobile paper, scrolled mobile paper, UK timezone default

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[MaxGhenis]

Superseded by #47, which squash-merged all the commits from this PR (header reveal fix, mobile header tweaks, favicon, ACA-from-scoring removal, hardest household segments removal, frontend flow + ranking defaults, linked program filters, rendered paper snapshot hash bump) along with three follow-ups on top: household-facts modal, Within 1% as the paper's headline metric, and DeepSeek V4 Pro/Flash on the public leaderboard.

Thanks for the work, @daphnehanse11 — closing this since the diff has already landed via #47.