Fix: add confidence threshold and prevent UI freezing on empty states

[khushanuma-shabbir]

Summary

This PR improves real-time prediction stability and fixes a bug where the
UI freezes when a hand leaves the camera frame.

Problem

Two issues existed in the prediction pipeline:

1. No confidence threshold — The model accepted the top prediction even
   at very low confidence (e.g., 15%). During hand transitions or random
   movements, the UI rapidly flickered with incorrect letters, and the
   5-second auto-add timer could trigger erroneously.

2. UI freezing on empty state — When a hand left the frame,
   detector.findHands() returned empty and the backend responded with
   {"success": False}. The frontend ignored this response, leaving the
   last detected letter permanently frozen on screen.

Changes Made

app.py

• Added CONFIDENCE_THRESHOLD = 0.70 constant (70% minimum confidence)
• Predictions below threshold now return "Unsure" and reset the
  auto-add timer, preventing false letter triggers
• When no hand is detected, now returns success: True with "Waiting..."
  so the frontend clears the screen properly
• Fixed edge case: if hand crop is empty (imgCrop.size == 0), state
  variables are now reset before returning

templates/index.html

• Updated JS to display "Waiting..." and "Unsure" cleanly without
  appending (0%) to the string

Testing Steps

1. Run python app.py and open http://127.0.0.1:5000
2. Hold a clear ASL sign — verify it shows the letter with confidence %
3. Slowly blur or rotate your hand — verify it shows "Unsure" instead
   of a random letter
4. Remove your hand from the frame entirely — verify screen clears to
   "Waiting..." immediately instead of freezing
5. Hold a valid sign for 5 seconds — verify the auto-add still works
   correctly for high-confidence predictions

Why This Improves the Project

• Eliminates false positives during hand transitions
• Makes the auto-add delay more reliable (only counts stable, high-confidence signs)
• Ensures users always know the app state (Waiting / Unsure / Confident)
• Prevents confusion from a frozen UI after moving hand out of frame