Fix iOS placeholder contrast on custom surfaces

[shanerbaner82]

iOS input placeholders can disappear on a custom dark surface while the device uses light appearance. Without an explicit placeholder override, SwiftUI uses its system color instead of the field's text color.

Default placeholders to the field text color at 60% opacity, while preserving explicit overrides such as on-input. Apply the same resolution to secure, revealed, selection-enabled, and multiline inputs. Android is unchanged.

Validation:

• Two focused regression checks pass (9 assertions); Pint and diff checks pass.
• Updated SwiftUI expressions type-check against the installed iOS simulator SDK.
• The equivalent fallback patch in Curbshot's installed plugin built and launched on the iPhone 18 Pro simulator. Visual confirmation is still pending; the complete plugin test suite was not run locally.