test : added unit tests for goals/share.ts

[tmdeveloper007]

Closes #2831.

Summary of What Has Been Done: Added vitest unit tests for the goal-sharing utilities in src/lib/goals/share.ts, covering getGoalProgressPercent, buildPublicGoalSharePath, and buildPublicGoalShareUrl.

Changes Made:

• Test getGoalProgressPercent: zero/negative/Infinity/NaN inputs return 0, correct percentage for simple values, rounding behavior, caps at 100, floors at 0
• Test buildPublicGoalSharePath: normal inputs, URL encoding for special chars in username and goalId
• Test buildPublicGoalShareUrl: correct concatenation, URL encoding for special chars

Impact it Made: These helpers are used in the goal-sharing feature. Test coverage prevents regressions in progress calculation and URL building for usernames with special characters.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus