Feature/develop 74 improve user experience

[jose-rZM]

• Improves input tolerance and feedback across LL(1) and SLR(1) tutors (numeric parsing, symbol lists, separators, duplicates).
• Adds dynamic placeholders per state to guide expected formats without extra UI noise.
• Preserves user input when answers are incorrect and the state doesn’t change.
• Normalizes LL(1) table cell input and hardens SLR(1) table parsing.
• Replaces the end-of-exercise modal with chat buttons to export PDF or exit.
• Solved SIGSEV caused by Feature/develop 66 symbol management #73 when running ll1.ll1_t_.at(EPSILON)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud