feat([issue-912]): in-lightbox regen controls (strength + optional prompt)

[atomantic]

Summary

Makes the SynthID-defeat Regenerate action tunable from the lightbox UI (follow-up to #958/#970), plus an empirically-driven floor change.

• In-lightbox controls — the Regenerate button now opens an inline panel with a strength slider (lower = more faithful) and an optional prompt field, so a pass can be dialed in without curl. Slider bounds come from the server (GET /regen/availability now returns strengthMin/Max/Default) so the UI stays in lock-step with route validation.
• Optional prompt — the route accepts an optional prompt (threaded to buildRegenParams as promptOverride; empty = the minimal-mutation default), so the same control supports a creative re-roll.
• Lower floor 0.1 → 0.02 — empirical sweep on this machine: across strength 0.02–0.25 the change is a flat ~8% (the VAE round-trip's own reconstruction error — the irreducible minimum for this approach), so minimal processing is already reached at the low end. Strength 0.0 is excluded: mflux special-cases it as "ignore the init image" and produces a near-random ~49%-changed image, which defeats the purpose.

Refs #912

Test plan

• routes/imageGen.test.js — availability endpoint now also asserts the strength bounds (strengthMin > 0, default within bounds); existing regenerate route tests still green.
• regen.test.js — unchanged behavior for buildRegenParams/clamp; 80 server tests across the two affected files pass.
• Client variants/normalize tests pass (30); client lint clean (0 errors); npm run build succeeds.
• Manual: empirical strength sweep (0.0/0.02/0.05/0.10/0.15/0.20/0.25) confirmed the flat ~8% floor and the degenerate 0.0 case; user's SynthID detector confirmed 0.25 and 0.4 defeat the watermark (lower values pending live testing via the new slider).