feat(theme): readable theme colors via useColors() + inline style

[MarJose123]

Summary

Fixes low-contrast, theme-ignoring colors across the app. Brass accent colors were used as
text/fills at 2.6–2.9:1 on light surfaces, CTA button labels were near-black on mid-brown
(muddy), the enabled toggle barely read against light cards, and several screens (onboarding
PIN, lock) were hardcoded to a single palette. Additionally, Tailwind color classes don't
follow the runtime dark/light theme in this project, so all colors now come from
useColors() + inline style.

What changed

New theme tokens (src/types/theme.ts, src/theme/, src/global.css)

Token	Light	Dark	Purpose
brassText	#8F5636 (5.2:1)	#C87B54	Brass as text/label (was 2.6–2.9:1 ❌)
brassButton	#A3542E	#C87B54	CTA button fill
buttonLabel	#F3EEE1 cream (4.7:1)	#1B1F1A ink	CTA button text

UI updates

• CTA buttons (Continue, Save, Add account, Update Now, …): light mode = deep brass +
  cream labels (button also stands out 4.8:1 vs the page). Dark mode unchanged (ink on brass,
  5.1:1).
• Toggle: enabled track uses brassText; thumb is parchment — reads clearly on light cards.
• FAB "+" in the bottom nav matches the CTA labels (cream in light, ink in dark).
• Lock / PIN screens: /lock and the onboarding PIN setup now follow the theme (previously
  hardcoded light); onboarding stack background too.
• Splash, ErrorBoundary, ReceiptScanResult: themed consistently.

Convention change (AGENTS.md)

• Documented that Tailwind color classes do not follow the runtime theme (they resolve
  against the :root light values; Appearance.setColorScheme() doesn't flip the evaluated
  media query).
• New rule: all colors via useColors() + inline style; className only for non-color
  layout. Hard Rule #15 updated; §8.1–8.6 and §8.10 rewritten.

Full sweep (~80 sites, 25 files)

• Converted every remaining color class (bg-surface-bg, text-text-primary, text-brass*,
  border-hairline, text-rust, …) to inline styles.
• Fixed latent bugs where files imported the static dark palette (edit-account,
  settings SectionLabel, ReceiptScanResult, receipt-scan) — labels rendered dark
  values in light mode.
• CategoryRingCard percent text was ink-on-dark in dark mode → textPrimary.

Verification

• tsc --noEmit ✅
• eslint src ✅ (prettier clean)
• bun test: 70 pass / 14 fail — identical to main baseline (pre-existing environment
  issue: react-native flow types under Bun; no regressions)

Notes

• No DB/schema changes; no new dependencies.
• SlothAppIcon brand SVG untouched (static artwork, not a theme color).
• Camera overlay caption keeps its module-level stylesheet color (drawn over the camera scrim).