fix(frontend): full-bleed layout, sticky top band and constrained scrolling

[theodubus]

Motivation

• Make the top band and main workspace span the full viewport width with no vertical gap so the UI touches the screen edges.
• Keep the top band fixed in place when the user scrolls, and prevent the whole page from scrolling.
• Constrain scrolling to the PDF viewer column only so the right tools column remains static and only the PDF content can scroll.

Description

• Update global layout and scrolling by changing frontend/src/styles.css to set html/body/#root to full size with overflow: hidden, make .topBand sticky, remove gutters and vertical gap, and allow only the viewer column to scroll while the tools column is non-scrollable.
• Rework frontend/src/App.tsx to a top-band + two-column structure, add a hidden shared file input and drag-and-drop upload area, and wire a file-change button into HeaderBar.
• Enhance viewer and selection plumbing in frontend/src/components/PdfViewer.tsx to compute per-page scales, emit selection rectangles via a selectionchange handler, and render preview rectangles; add selection lifecycle handling in the rules UI.
• Propagate selection rects to the backend by adding RectInput and including rects in the payload in frontend/src/api.ts, and add UI/locale strings and small UX refinements across rules components (RulesSection, RulesList, EditRuleModal).

Testing

• Built the frontend with cd frontend && npm run build and the production build completed successfully.
• Started the dev server with cd frontend && npm run dev -- --host 0.0.0.0 --port 4173 and Vite reported readiness.
• Captured a Playwright screenshot validating the full-bleed + sticky header layout and viewer-only scroll behavior (artifact created).

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Codex Task