update help documentation and re-enable hints in settings

app/src/main/assets/hints/exercise_hint.md

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+## What Are Exercises?
+
+Exercises help you practice and memorize your vocabulary through interactive activities. You can practice with flashcards, test your spelling, or challenge yourself with word puzzles.
+
+## Exercise Types
+
+### Flashcard Mode
+
+A simple card shows one word, and you guess the translation. Tap to flip and reveal the answer, then mark it as correct or wrong.
+
+### Spelling Practice
+
+A word appears and you must type the translation yourself. This is great for memorizing spelling and reinforcing what you've learned.
+
+### Multiple Choice
+
+Four answer options are shown and you pick the correct translation. Good for quick recognition practice.
+
+### Word Jumble
+
+Letters are scrambled and you must click them in the correct order to spell the word. Excellent for mastering spelling.
+
+## Starting an Exercise
+
+1. **From the Dashboard** - Tap the Start Exercise button
+2. **From a Category** - Open any category and tap Start
+3. **From the Main Screen** - Tap the floating action button
+
+## Before You Start
+
+You can customize your exercise:
+
+- **How many cards** - Use the slider to pick 1 to 100+ cards
+- **Quick select** - Tap 10, 25, 50, or 100 for fast selection
+- **Language direction** - Choose which language to translate from and to
+- **Exercise types** - Pick one type or mix several together
+- **Shuffle cards** - Randomize the order
+- **Shuffle languages** - Randomize which side (word/translation) is shown
+- **Training mode** - Practice without affecting your progress
+- **Due today only** - Only practice items scheduled for today
+
+## Training Mode
+
+When training mode is ON:
+- Your answers won't affect your progress stages
+- No statistics are recorded
+- Perfect for casual practice or testing yourself
+
+When training mode is OFF (default):
+- Correct answers may advance items to higher stages
+- Progress is saved and tracked
+- Affects your learning statistics
+
+## During an Exercise
+
+The progress bar at the top shows:
+- **Green** - How many you've gotten right
+- **Red** - How many you've gotten wrong
+- **Total** - How many cards in this session
+
+Tap the close button anytime to exit (you'll be asked to confirm).
+
+## After Finishing
+
+Your results screen shows:
+- Your overall score as a percentage
+- How many you got right and wrong
+- Total cards practiced
+
+You can then:
+- **Start Over** - Begin a fresh exercise
+- **Repeat Wrong** - Practice only the cards you missed
+- **Finish** - Return to the main screen
+
+## Tips
+
+- **Mix it up** - Combine different exercise types for variety
+- **Daily practice** - Use "Due Today Only" for regular reviews
+- **Focus on mistakes** - Use "Repeat Wrong" to learn from errors
+- **Start small** - Begin with 10-25 cards and increase over time
+- **Use training mode** - When you want to practice without pressure