fix: replace hardcoded progress cap with derived PROGRESS_MAX_POINTS

[nancysangani]

Fixes #738

computeProgressPoints used a hardcoded divisor of 250 that was lower than what a normal user accumulates, producing percentages above 100% before the Math.min clamp. The clamp fixed the display but left aria-valuenow set to values like 150, violating the ARIA progressbar spec.

Changes:

• static/script.js — extracted per-action point values and activity targets as named constants; PROGRESS_MAX_POINTS (450) is derived from those constants; computeProgressPoints clamps progress.points at max; percentage formula uses PROGRESS_MAX_POINTS as divisor
• tests/test_progress_calculation.py — 28 tests: arithmetic correctness, original bug scenario regression, clamp behaviour, ARIA attribute validation, accessibility range checks

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[nancysangani]

Hi @komalharshita, please review the PR when you get a chance. Thanks!