[Frontend] Mobile-responsive layout for all pages

[Gbangbolaoluwagbemiga]

Problem

SecureFlow is desktop-first. Several pages break on small viewports — content overflows, cards don't stack, milestone timelines are unreadable on < 640px.

Pages to fix (test at 375px)

• `HomePage.tsx` — hero section, CTA buttons
• `JobsPage.tsx` — job card grid → single column
• `CreatePage.tsx` — multi-step form, milestone inputs
• `DashboardPage.tsx` — stats row → 2×2 grid
• `FreelancersPage.tsx` — freelancer cards
• `ApprovalsPage.tsx` — action buttons
• Navigation — hamburger menu
• `ChatDialog` — full-screen on mobile

Guidelines

• Use Tailwind responsive prefixes (`sm:`, `md:`, `lg:`)
• Test in Chrome DevTools device toolbar
• Lighthouse mobile score ≥ 70

Good first issue: Pick one page, make it responsive, open a PR.