fix(app): make sheet header search text readable in dark mode

The model picker search input rendered text in the light-theme
foreground color on a dark surface, leaving typed characters invisible.

@gorhom/bottom-sheet mounts header subtrees before the sheet is visible
and keeps them mounted across theme changes, so any color read from
StyleSheet.create((theme) => ...) at the caller goes stale. Move text
color and placeholder color into AdaptiveTextInput itself via
withUnistyles, with the leaf's color appended last so callers cannot
re-introduce a stale read. Drop the now-redundant placeholderTextColor
props at the header search call sites.

Also ban useUnistyles() in docs/unistyles.md. The hook subscribes to
every runtime change and forces a re-render each cycle; new code uses
withUnistyles or StyleSheet.create instead.

packages/app/src/components/ui/combobox.tsx

@@ -194,7 +194,6 @@ export function SearchInput({
           // @ts-expect-error - outlineStyle is web-only
           style={SEARCH_INPUT_STYLE}
           placeholder={placeholder}
-          placeholderTextColor={theme.colors.foregroundMuted}
           initialValue={value}
           resetKey={resetKey}
           value={value}