Add years of experience to Profile

Goal

Track the user's total years of relevant professional experience in the Profile.

Requirements

• Calculate/display total years of experience.
• Prefer deriving this from structured work experience where possible.
• Handle overlapping employment periods correctly.
• Define how internships, freelance work, military/service, and other experience types are treated.
• Allow manual correction/override if necessary.

Acceptance Criteria

• Profile displays years of experience.
• Calculation is deterministic.
• Overlapping jobs are not double-counted.
• Tests cover common edge cases.