feat(frontend): step 1 UI layout refactor (top band + two-column workspace)

[theodubus]

Motivation

• Remove the boxed/card confinement for the viewer/tools area and replace it with a top horizontal band plus a two-column workspace (wide viewer + narrower tools) to match the requested layout change.
• Replace the legacy placeholder text shown before a PDF is loaded with an actionable upload area (drag-and-drop + CTA) and expose a single, simpler header with language toggles and a left-side "load new PDF" button when a file is present.
• Surface selection-based redaction as a first-class UI flow so users can create selection rules from text selected in the viewer.

Description

• Implement top band and two-column layout by adding a new HeaderBar and reworking App.tsx layout, and update styles in styles.css to provide the new .topBand, .mainColumns, .viewerPane, .toolsPane, and upload dropzone visuals.
• Move file-loading UX into the viewer area with a hidden shared file input, a click-to-choose button and a drag-and-drop zone, and show a top-band Load new PDF button (localized) when a file is loaded; code changes live in frontend/src/App.tsx, frontend/src/components/HeaderBar.tsx and localized keys in frontend/src/locales/*.json.
• Extend the viewer and selection plumbing in frontend/src/components/PdfViewer.tsx to compute per-page scales, expose user text selections via a selectionchange handler producing UiRect[], and draw preview rectangles in the viewer; added selection rule support and utilities in frontend/src/types/uiRules.ts and frontend/src/components/*.
• Add rects support to the API client by introducing RectInput and passing selection rects in the payload (frontend/src/api.ts), and update rules UI (RulesSection.tsx, RulesList.tsx, EditRuleModal.tsx) to surface the selection workflow and prevent editing of selection-type rules.

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 confirmed Vite reported readiness.
• Captured an automated Playwright screenshot of the new empty-state layout at artifacts/ui-layout-step1-empty.png to validate the upload/dropzone and two-column arrangement.

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Codex Task