feat(#35): add responsive hamburger menu and fix sidebar visibility

[hectorguillen-blip]

Description

Adds a responsive hamburger menu to the admin tool header and fixes the sidebar navigation visibility on narrow viewports.

Closes #35

Changes

Hamburger menu in header

• Added a hamburger button (≡) to the header that appears on narrow viewports (mobile/tablet)
• Clicking it toggles the app link and logout button, which are hidden by default on mobile
• Toggle state is managed locally in HeaderComponent via menuOpen boolean
• New header.component.less added with mobile-only hamburger styles

Sidebar always visible

• Fixed the sidebar disappearing on narrow viewports — it now stays in place at all screen sizes
• Sidebar shrinks naturally as the viewport narrows, allowing nav link text to wrap onto multiple lines
• Removed the old position: fixed overlay behaviour from sidebar.component.less
• Updated common.less with flex-wrap: nowrap and flex-shrink: 1 to keep sidebar inline

Checklist

• Readable — follows style guide, ESLint passing
• UI/UX backwards compatible — sidebar and header behaviour matches the original tool at all viewport sizes

[jomcarvajal]

LGTM