fix(frontend/backend): stabilize text selection and add full-page redaction support

[theodubus]

Motivation

• Prevent unstable and flickering text selection in the PDF viewer caused by frequent rerenders when page-size callbacks change reference.
• Support user-driven "full page" redaction rules in the UI and ensure backend redaction logic does not over-tighten full-page rectangles.

Description

• Memoize page-size updater in App with useCallback and pass a stable onPageSizeChange to PdfViewer to avoid rerender churn during drag selection.
• Enhance PdfViewer to report the current page (onCurrentPageChange), observe visible page via IntersectionObserver, call onPageSizeChange once per page load, and include page-level preview highlighting for page rules.
• Add UI and types for page rules: UiRule now supports kind: "page", RulesSection exposes a "Redact page" button, RulesList and EditRuleModal were adjusted to handle page rules, and localization keys were added.
• Propagate full-page application options through the frontend API payload with options.apply_images and options.apply_graphics.
• Backend safety: _tighten_rect_vertical now skips aggressive vertical tightening for very large rectangles (height > 24pt) to avoid compressing full-page redactions.
• Add an integration test test_redact_rectangles_full_page_rect_redacts_target exercising full-page rectangle redaction behavior.

Testing

• Successfully built the frontend with npm run build (TypeScript + Vite) and the build completed without errors.
• Started the dev server with npm run dev and verified the app by capturing a browser screenshot via a Playwright script.
• A new backend integration test was added for full-page redaction, but no backend test suite was executed in this run.

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Codex Task