fix(reader): add desktop PDF zoom controls

[codedogQBY]

Analysis

• [Suggestion] pdf的阅读体验问题 #363 reports that Windows desktop PDF pages cannot be zoomed in or out, which blocks scanned/small-text PDFs.
• The fixed-layout renderer already supports fit-width/fit-page and absolute numeric zoom, but absolute zoom would replace the default auto-fit behavior. This PR adds a relative zoom factor on top of the existing fit scale.
• PDF浏览方式优化 #87 also asks for broader PDF browsing improvements. This covers the zoom portion and keeps overflow panning available, but does not close the continuous-scroll request.

Changes

• Add a persisted fixedLayoutZoom reader setting for PDF/CBZ fixed-layout books.
• Show fixed-layout zoom controls in the desktop reader toolbar: zoom out, current percentage, zoom in, and reset.
• Pass zoom-factor from FoliateViewer into foliate-js and reapply renderer settings when it changes.
• Teach FixedLayout to multiply fit-width/fit-page scale by the zoom factor while leaving absolute numeric zoom unchanged.
• Add English and Chinese toolbar labels.

Verification

• pnpm --filter app exec tsc --noEmit
• git diff --check
• node --check packages/foliate-js/fixed-layout.js
• pnpm exec biome check packages/app/src/components/reader/ReaderToolbar.tsx packages/core/src/types/book.ts packages/core/src/i18n/locales/en/reader.json packages/core/src/i18n/locales/zh/reader.json packages/foliate-js/fixed-layout.js
• pnpm --filter app build

Note: a broader Biome check including ReaderView, FoliateViewer, and settings-store still reports pre-existing lint diagnostics unrelated to this PR.

Fixes #363
Related #87