fix: consolidate theme state on next-themes useTheme()

[Nife-tanny]

Overview

Resolves #1262 by consolidating theme state management on next-themes' useTheme() as the single source of truth, eliminating two independent implementations that read/wrote the same flowfi-theme localStorage key without coordinating with each other or with the ThemeProvider.

What Each System Did Before

1. layout.tsx — Used ThemeProvider (wrapping next-themes) with storageKey="flowfi-theme". This was the correct, canonical implementation.

2. useSettings.ts — Had its own setTheme() callback that independently:

   • Updated module-level sharedSettings.theme state
   • Wrote to localStorage with key "flowfi-theme"
   • Manually toggled the dark class on document.documentElement
   • Exposed getStoredTheme() and applyStoredTheme() helpers

3. settings-content.tsx — Had its own useState for theme initialized from localStorage.getItem("flowfi-theme"), and a toggleTheme() function that independently:

   • Set local React state
   • Wrote to localStorage with key "flowfi-theme"
   • Manually toggled the dark class on document.documentElement

The bug: None of the three called each other's setters, so toggling theme in one surface didn't reflect in another within the same tab — only a page reload or cross-tab storage event synced them.

What Changed

useSettings.ts

• Removed Theme type, theme from Settings interface and DEFAULT_SETTINGS
• Removed theme from STORAGE_KEYS
• Removed setTheme() callback from the hook return
• Removed getStoredTheme() and applyStoredTheme() helpers
• Removed theme-related logic from loadSettingsFromStorage()
• Preserved untouched: displayCurrency, amountFormat, decimalPlaces settings and all their setters, formatAmountWithPreference(), getDecimalPlaces(), getAmountFormat(), getDisplayCurrency()

settings-content.tsx

• Replaced local useState for theme + manual toggleTheme() with useTheme() from next-themes
• All theme buttons now call setTheme() from useTheme(), ensuring they share state with the provider
• Preserved untouched: displayCurrency, amountFormat, decimalPlaces local state and their UI

useSettings.test.ts

• Removed theme-related test cases (the hook no longer returns theme or setTheme)
• Updated the cross-tab sync test to use displayCurrency instead of theme
• Preserved untouched: All formatAmountWithPreference tests, all non-theme settings tests

No changes to:

• layout.tsx — already correctly configured as the ThemeProvider
• ModeToggle.tsx — already correctly using useTheme() from next-themes
• theme-provider.tsx — already correctly wrapping NextThemesProvider

Same-Tab Sync Verification

After this change, all three surfaces share the same reactive state via useTheme():

• ModeToggle (navbar): calls setTheme() from useTheme() → next-themes updates its React context → all subscribers re-render
• settings-content.tsx: calls setTheme() from useTheme() → same path as above
• Any useSettings consumer: no longer has theme state at all → no conflict possible

Since useTheme() returns values from the ThemeProvider's React context, any call to setTheme() immediately updates all other components reading from useTheme() in the same render tree — no page reload needed.

Pre-Existing Failures (Intentionally Untouched)

• create-stream-recipient-prefill.test.tsx: 1 test fails with a 5-second timeout. Confirmed pre-existing (fails identically on the base branch before any changes).
• TypeScript errors: All tsc --noEmit errors are in pre-existing test files (batch-stream-wizard.test.tsx, streams.test.ts, useStreamingAmount.test.tsx, etc.) — none in the files modified by this PR.
• Next.js build: Turbopack panics with Invalid distDirRoot: ".next" — confirmed pre-existing (same error on base branch).

Local Verification

Check	Status
npm run test (vitest)	✅ 322/323 pass (1 pre-existing timeout)
ESLint on changed files	✅ Clean
tsc --noEmit	✅ No new errors (all errors pre-existing in other files)
next build	⚠️ Pre-existing Turbopack error on base branch

Closes #1262

[K1NGD4VID]

resolve conflict on this issue @Nife-tanny