Add error feedback to UI (competitors, settings, runs)

[jluk]

Problem

Multiple pages silently swallow errors — user gets no feedback when operations fail:

• Competitor add/edit/delete: errors logged to console only
• Settings save: catch block shows "Saved" even on failure
• Run failures: shows "Failed" text but no error details

Goal

Add user-visible error messages (inline or toast) when operations fail. Users should always know when something didn't work and why.

Files affected

• src/app/competitors/page.tsx (lines 122, 162, 178)
• src/app/settings/page.tsx (lines 101-105)
• src/app/runs/page.tsx (lines 190-199)