No theme system — hardcoded colors and fixed font sizes break OS accessibility text scaling

[ndii-dev]

Every screen (welcome.tsx, create.tsx, import.tsx, home.tsx) inlines the same hardcoded hex colors (#0f172a, #3b82f6, etc.) per-file with no shared theme module, and every fontSize is a fixed number with no respect for the OS's Dynamic Type (iOS) / font scale (Android) accessibility settings — meaning a low-vision user who has increased their system text size site-wide gets no benefit in this app at all.

Definition of done:

• Centralized theme/token module consumed by all screens, replacing inline hex literals
• Text respects allowFontScaling (React Native's default) rather than any explicit allowFontScaling={false} override, verified none exist and none get added
• Manually verified layouts don't break/clip at 200% OS text scale

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This is not a starter-issue. The Definition of done above is the full
acceptance criteria, not a subset to sample from — a PR that addresses part
of it is an unfinished issue, not a smaller one. Your PR must include, in
the PR description itself:

• Root cause / design-decision rationale in your own words — not a restatement of this issue
• Every Definition of done bullet above addressed explicitly, with a one-line note on how
• Evidence the code actually runs: pasted test/build output, a screen recording or before/after screenshots for UI changes, or real (non-mocked) logs for network/contract-facing work
• New or updated tests included and shown passing (paste the output)
• Any adjacent/related behavior this issue calls out re-verified, not assumed unaffected

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