refactor: remove frequency chart from list of letters, sounds, words

[jo-elimu]

Issue Number

• Resolves Remove content frequency charts #2358

Purpose

Technical Details

Testing Instructions

Screenshots

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Walkthrough

This PR removes frequency chart rendering from the letter list page to improve page load performance. Changes include updating Maven dependency versions, removing StoryBookParagraphDao dependency and letter frequency calculation logic from the backend controller, and removing the Chart.js visualization block from the JSP view.

Changes

Cohort / File(s)	Summary
Maven Dependencies pom-dependency-tree.txt	Updated webapp version from 2.6.142-SNAPSHOT to 2.6.148-SNAPSHOT and model transitive dependency from 2.0.127 to 2.0.128.
Backend Controller src/main/java/ai/elimu/web/content/letter/LetterListController.java	Removed StoryBookParagraphDao import, field declaration, and associated logic for reading paragraphs, computing letter frequency maps, and exposing frequency data to the model.
Frontend View src/main/webapp/WEB-INF/jsp/content/letter/list.jsp	Removed Chart.js script import, canvas element, and chart initialization code; CSV export and table rendering logic remain intact.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch 2358-remove-content-frequency-charts

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 16.80%. Comparing base (9a7ef0e) to head (8db25ba).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2359      +/-   ##
============================================
- Coverage     16.83%   16.80%   -0.03%     
+ Complexity      465      464       -1     
============================================
  Files           264      264              
  Lines          7852     7836      -16     
  Branches        903      899       -4     
============================================
- Hits           1322     1317       -5     
+ Misses         6454     6445       -9     
+ Partials         76       74       -2     

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