fix: repair all tasks group rendering

[saurabhhhcodes]

Related Issue

Closes #447

Summary

Fixes the All Tasks rendering crash by restoring the missing string concatenation between the Due soon and This week task groups.

Changes Made

• Added the missing + operator between adjacent renderGroup() calls in renderTasks().
• Kept the change scoped to the reported crash path in js/app.js.

Testing

• node --check js/app.js
• git diff --check

GSSoC Labels Requested

Please add gssoc:approved, level:beginner, quality:clean, and type:bug if this fix is accepted.

Screenshots

Not applicable; this is a one-line runtime/syntax fix.

Checklist

• Code follows project style
• Tested locally
• No unrelated changes included
• Documentation updated (if applicable)

[saurabhhhcodes]

Hi @Charushi06, this one is a very small crash/syntax fix for #447 and GitHub currently shows the branch as clean/mergeable. I validated it with node --check js/app.js and git diff --check. If it looks good, could you please review/merge and add the GSSoC labels from the PR body (gssoc:approved, level:beginner, quality:clean, type:bug)?