Fix landing page responsiveness for mobile viewports

[SpaleRuby]

Fixed issue #3 with a focused mobile responsiveness pass on the landing page.

Changes include:

• Added mobile navigation with a collapsible menu for small screens.
• Added responsive CTAs (full-width on narrow screens) and safer paddings.
• Switched tools grid to 1→2→4 columns for narrow breakpoints.
• Added mobile-specific overflow guard and tighter spacing to avoid horizontal overflow.
• Kept desktop layout behavior unchanged where applicable.

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