fix: resolvedTheme from useTheme() now reflects forcedTheme

[i8ramin]

Summary

useTheme().resolvedTheme ignores forcedTheme — consumers get the user's stored preference instead of the active forced theme.

Closes #252

The Bug

When forcedTheme is set on ThemeProvider, the DOM is updated correctly (the data-theme attribute and color-scheme style reflect the forced value), but the React context value for resolvedTheme is wrong:

// _app.tsx
<ThemeProvider forcedTheme="light">

// SomeComponent.tsx
const { resolvedTheme } = useTheme()
// resolvedTheme === 'dark'  ← WRONG (user's stored preference)
// expected: 'light'          ← the forced theme

This forces consumers to read forcedTheme directly and fall back manually:

const { resolvedTheme, forcedTheme } = useTheme()
const effectiveTheme = forcedTheme ?? resolvedTheme // workaround

Root Cause

In next-themes/src/index.tsx, the provider value computes:

resolvedTheme: theme === 'system' ? resolvedTheme : theme,

This ignores forcedTheme entirely. The theme state variable holds the user's stored preference, not the forced value.

Fix

One-line change — prepend forcedTheme with nullish coalescing and correct parentheses:

resolvedTheme: forcedTheme ?? (theme === 'system' ? resolvedTheme : theme),

Note on the v1 branch

The v1 branch attempted this same fix but has an operator precedence bug — the ternary is not wrapped in parentheses:

// v1 branch (line ~228):
resolvedTheme: forcedTheme ?? theme === 'system' ? resolvedTheme : theme,

// Evaluates as:
(forcedTheme ?? (theme === 'system')) ? resolvedTheme : theme
// NOT:
forcedTheme ?? (theme === 'system' ? resolvedTheme : theme)

This PR uses the correct precedence.

Test

Added a Playwright test that verifies resolvedTheme from useTheme() matches the forced theme on forced-theme pages (the existing tests only checked DOM attributes, which is why this bug wasn't caught).

Impact

• Minimal — one line changed in the provider value computation
• No breaking changes — forcedTheme is still available as a separate field
• DOM behavior unchanged (was already correct via applyTheme)
• Only the React context value is fixed