feat: EXIF auto-detection, azimuth constraint filter, and loupe magnifier

[nickFalcone]

Summary

• EXIF extraction: Automatically reads timestamp and compass bearing from photo
  metadata on upload. Prefers GPS timestamp (UTC), falls back to camera timestamp +
  UTC offset, with a manual offset picker for photos without timezone data.

• Azimuth constraint: Combines the EXIF compass bearing with the shadow pixel
  direction to compute the sun's azimuth. Applied as a toggleable post-analysis filter
  in the Analysis Panel — no re-computation required. Narrows the shadow band from a
  global ring to a candidate region (±10° tolerance). Disabled automatically for
  magnetic bearings. Works in both single and dual-photo intersection modes.

• Loupe magnifier: CSS-only 3× magnifier follows the cursor during point marking,
  with cyan crosshairs for sub-pixel precision. Hides once all 3 points are placed.

Test plan

• Upload a photo with GPS EXIF — date/time fields should pre-fill, green GPS badge shown
• Upload a photo with UTC offset EXIF — date/time pre-filled, blue badge shown
• Upload a photo with compass bearing — azimuth section appears, toggle enabled after marking all
  3 points
• Upload a photo with magnetic bearing (compassRef = M) — toggle shown but disabled with
  explanation
• Upload a photo with no compass bearing — toggle shown but disabled with "No compass bearing"
  note
• Hover over image while marking points — loupe magnifier appears; disappears after 3rd point
• Run analysis with azimuth constraint on — 🧭 badge appears on map
• Toggle azimuth off — map reverts to full band without re-analyzing
• Replace image — azimuth constraint clears and toggle resets