fix(camera): keep zoom slider off the ultra-wide lens

[farfromrefug]

Summary

• The zoom slider is now driven by a logarithmic 0..100 scale instead of raw zoom ratios. On Android slider is an android.widget.SeekBar whose progress is an int, so raw ratios were truncated: on a device reporting a minimum zoom of 0.7 (ultra wide lens) the only reachable values were 0.7 / 1.7 / 2.7 … — 1x could not be selected, and 0.7x was the easiest value to land on.
• The scale snaps to the camera neutral zoom, with a snap window derived from the device range (slightly wider than half a slider step), so an exact 1x is always reachable whatever the device reports.
• The zoom stored in defaultZoom is clamped to the range of the current device on camera open, and the ratio reported by the native pinch handler (which is not coerced by the plugin) is clamped before being persisted. A stale or out of range value can no longer pin the camera to the ultra wide lens on every launch.
• Mapping and clamping live in a new UI free app/utils/cameraZoom.ts so they can be unit tested.

Testing

• npx vitest run app/utils/cameraZoom.test.ts — round trip, neutral snap, exact 1x reachability over several device ranges, clamping of out of range and non finite values, degenerate ranges.
• yarn svelte-check — clean.
• Still needs a visual check on Android, ideally on a device with a sub 1x minimum zoom (Pixel 7): camera opens at 1x, the floating label tracks the thumb, sliding reaches exactly 1x, and the chosen zoom survives a relaunch.

Refs #693