fix: dashboard & inner app pages UI cleanup (closes #118)

[prasanna-0806]

Part of #116

🎯 Summary

Focused UI cleanup pass across all inner app pages — spacing, typography, dark mode, and mobile layout. No redesigns, no data fetching changes.

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✅ Spacing

• Section padding consistent — all pages now use py-8–py-12 for section padding. Removed arbitrary mt-5, px-3 usages across /events/[id], /submit-project, /blog
• Card padding consistent — all cards use p-6. Fixed /blog cards which were using p-5
• Form fields consistent gap — label → input → helper text uses gap-4 / space-y-2 consistently across /onboard, /profile, /submit-project

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✅ Typography

• <h1> consistent — all inner app pages use text-3xl font-bold for page headings (/dashboard, /profile, /onboard, /blog/[slug])
• Body text 16px minimum — removed sub-16px body copy. text-xs / text-sm now only used on labels, helper text, and metadata (author, date)
• Muted text consistent — replaced arbitrary text-gray-400, text-gray-500, opacity classes with text-muted-foreground token across all pages

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✅ Mobile layout (375px)

• /dashboard KPI cards — grid md:grid-cols-2 lg:grid-cols-3 collapses to single column cleanly at 375px

• /submit-project form — submit button changed to w-full md:w-auto, form inputs full-width, no clipping at 375px

• /events/[id] detail card — date and location changed from single rigid flex row to flex-wrap with separate <span> elements. CTAs full-width at mobile

• /blog/[slug] article — max-w-2xl px-4 container readable at 375px. Like button and comment form accessible on narrow screens

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✅ Dark mode

• No white-on-white or black-on-black — fixed:
  • /onboard form card was solid white in dark mode → dark:bg-zinc-900 with dark text throughout
  • /dashboard, /events, /blog/[slug] headings using text-brand-blue (hardcoded navy) now have dark:text-white
  • /events "Upcoming Events" count was navy-on-dark (invisible) → added dark:text-white

• Card borders visible in dark mode — replaced border-0 with border border-border on:
  • /events — stat cards, upcoming event cards, past event cards, empty state card, CTA card
  • /dashboard — action cards, GitHub integration card, getting started section
  • /blog — blog list cards
  • Screenshots from above

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Other changes

• Replaced all /raccoon.png references with /landing/logo.png site-wide: /blog, /blog/[slug], /dashboard, /onboard, /footer, /auth/login, /auth/sign-up, /docs, /learn, /not-found

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📝 Notes for reviewers

Blog image migration — any existing Supabase blogs records with the old raccoon image should be updated:

UPDATE blogs SET image = '/landing/logo.png' WHERE image = '/raccoon.png';

public.profiles table — the dashboard layout guard introduced in #145 queries public.profiles (with id and is_onboarded columns). This table must exist in Supabase for /dashboard to load correctly.

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✅ Acceptance criteria

• All in-scope pages visually consistent at desktop (1280px) and mobile (375px)
• Dark mode verified on all pages — screenshots attached
• No body text below 16px
• npm run build passes with no type errors
• Screenshots attached

Closes #118