Fix fire detection backend selection and simplify return type handling

[Copilot]

Applied code review feedback from PR #356. Fire detection logic was checking backend == "auto" but _get_detection_backend() only returns "local", "huggingface", or None, causing fire detection to be skipped when local backend was selected even with HF fallback available.

Changes:

• Fixed backend condition to backend == "huggingface" or (backend == "local" and ENABLE_HF_FALLBACK) to properly enable HF fallback from local mode
• Removed 15 lines of unnecessary type branching since detect_fire_clip consistently returns List[Dict]

# Before: checked for "auto" that never occurs, complex type handling
if backend == "huggingface" or backend == "auto":
    result = await detect_fire_clip(image)
    if isinstance(result, list): return result
    if isinstance(result, dict) and "detections" in result: ...
    # 10+ more lines of branching

# After: correct backend check, direct return
if backend == "huggingface" or (backend == "local" and ENABLE_HF_FALLBACK):
    if await self._check_hf_available():
        return await detect_fire_clip(image)

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Summary by cubic

Fixes SPA navigation glitches and corrects fire detection backend selection with HF fallback. Streamlines backend flow to make page transitions smooth and reduce server work.

• Bug Fixes

  • Keep navigation client-side and fix back/forward + active state for nested routes.
  • Use HF for fire detection when selected or when local with fallback; handle unavailable backends gracefully.

• Refactors

  • Consolidated handlers with early returns; batched common DB reads to cut redundant queries.
  • Simplified detect_fire to return List[Dict] directly from detect_fire_clip.

^{Written for commit 8b8d281. Summary will update on new commits.}