feat: add visual focal length simulator with real-time camera zoom

[JustCreature]

Summary

Adds an interactive focal length simulator that overlays a ruler-style slider on the camera preview, allowing photographers to visualize and simulate different
focal lengths (15mm-200mm) in real-time through digital zoom.

Key Features

• Visual Ruler Overlay: Semi-transparent slider at bottom of camera view with marks at common focal lengths (15, 24, 35, 50, 85, 135, 200mm)
• Real-Time Digital Zoom: Video preview zooms instantly as you adjust focal length
• Accurate Photo Capture: Photos capture only the zoomed portion matching the preview
• Letterbox Effect: Shows black bars for ultra-wide angles (<24mm) indicating FOV wider than camera can capture
• Persistent Settings: Remembers focal length across camera sessions
• Synced State: Slider and settings dialog share single source of truth (currentSettings.focalLength)

Mobile UX Improvements

• Larger touch targets (32px thumb, 36px when active)
• Step size of 5mm for easier control (less jumpy on touch)
• Visual feedback when dragging (darker overlay, larger focal length text)
• Smooth transitions on all interactions
• TextField allows clearing to type new values

Technical Implementation

New Component:

• FocalLengthRulerOverlay.tsx - Ruler overlay with Material-UI Slider

Updated:

• CameraScreen.tsx - Integrated zoom calculation, letterbox rendering, overlay positioning
• camera.ts - Updated captureImage() to accept zoomFactor and crop video frame accordingly
• FocalLengthSlider.tsx - Fixed TextField to allow empty state for easier value entry

Zoom Formula:
baseline = 24mm (iPhone standard camera)
zoomFactor = focalLength / baseline

Examples: 24mm=1.0x, 50mm=2.08x, 85mm=3.54x

User Benefits

• Visualize composition with different focal lengths before shooting
• Educational tool to understand focal length effects
• Better framing decisions for film photography
• No need to guess how a scene would look with different lenses

Testing

• ✅ All E2E tests pass (photography workflow, camera management)
• ✅ Build succeeds without errors
• ✅ State persistence verified
• Manual testing recommended on iPhone for touch interaction quality

Screenshots

Recommended: Add screenshots showing the ruler overlay at different focal lengths