Fix/create post dark mode

[xenacode-art]

Fixes #342

The Create Post page used hardcoded gray-* Tailwind classes that don't respond to the .dark theme class. This caused poor
contrast (light backgrounds, dark text) when dark mode was active.

Changes

src/app/(main)/(posts)/posts/createpost/page.tsx

Before	After
bg-gray-50	bg-common-background
border-gray-100	border-common-minimal
text-gray-700 (headings/labels)	text-dark-primary / text-text-primary
ring-gray-300	ring-common-contrast
bg-gray-100 (sidebar)	bg-common-background
bg-gray-200 (hashtag pills)	bg-common-minimal

All replacements use the existing semantic CSS variable tokens defined in globals.css that already switch between light/dark
values.

Test plan

• Toggle dark mode and open /posts/createpost
• Verify form card, labels, input rings, sidebar, and hashtag pills are all visible and properly themed
• Verify light mode is unaffected