fix(desktop): make lightbox zoom controls interactive

[kalvinnchau]

Summary

• Replace the decorative lightbox zoom icons with accessible Zoom out and Zoom in buttons.
• Route button clicks through the existing 1x–3x, 5% stepped zoom state without dismissing the viewer, including clicks on SVG icon descendants.
• Extract the zoom toolbar so markdown.tsx remains below the repository file-size threshold.

Testing

• pnpm build:e2e
• pnpm exec playwright test image-attachment-gallery.spec.ts --project=smoke — 10 passed
• pnpm typecheck
• Full desktop unit suite — 5,397 passed
• Pre-push checks — file-size gate, desktop checks, typecheck, and desktop tests passed
• git diff --check

UI evidence

The rendered lightbox flow is covered by the gallery smoke test: Zoom out is disabled at 100%, Zoom in changes the value to 105%, Zoom out returns it to 100%, and the dialog remains open throughout. Existing gallery, keyboard, spoiler, context-menu, and close-path assertions also pass.

[wpfleger96]

🤖 Combined review — two independent source reviews plus live headless E2E, all at head 57f76e6. No blocking findings.

MINOR — repeated 5% steps miss the displayed endpoint by one click due to floating-point accumulation. The button handlers add/subtract IMAGE_LIGHTBOX_ZOOM_STEP and rely on updateZoom → clampImageLightboxZoom, which clamps but never rounds. 40 Zoom in clicks from 1 yield 2.9999999999999973: the label renders 300% while disabled={zoom >= IMAGE_LIGHTBOX_MAX_ZOOM} stays false until click 41; the mirror case holds for Zoom out from an exact 3. Fix by snapping stepped values to the 5% grid before clamping (integer-percent math is simplest); that also lets the exact-endpoint disabled state be asserted after exactly 40 steps in the spec.

Everything else checked out under live E2E: buttons step exactly 5% without dismissing the dialog, direct clicks on nested SVG descendants work (the Element widening in the capture guards is the right call — SVG targets are not HTMLElement), keyboard Enter/ArrowRight behave, 100-click runs clamp and disable at both bounds, and tab order skips the disabled button. The extraction preserves slider behavior mechanically and keeps markdown.tsx under the size gate.

Cross-PR: textually conflicts with #6705 in desktop/src/shared/ui/markdown.tsx; whichever lands second must keep both ImageLightboxZoomControls and #6705's ImageGalleryStatus/openThreadImageGallery, then rerun the gallery smoke row.