fix: truncate username overflow in dashboard header on mobile (#1762)

[Bhavy12-cell]

Closes #1762

Fixed username text overflow in Dashboard header on mobile viewports.

Changes:

• Added truncate + max-w-[120px] sm:max-w-[200px] to username span
• Added min-w-0 max-w-full to greeting badge container
• Long names now truncate with ellipsis instead of overflowing
• No visual regression on desktop viewports

[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