`textarea` / `input` / `modal` / `markdown` component elements fail code generation in pure components (no receiver)

Description

Using a component element (<textarea />, <input />, <modal />, <markdown />) inside a pure component (one without a receiver) produces invalid Go code that fails to compile. Struct components (with receiver) work fine.

Steps to Reproduce

Create a .gsx file with:

package main

import tui "github.com/grindlemire/go-tui"

templ MyForm() {
	<textarea placeholder="Type here..." />
}

Run tui generate — the generated _gsx.go file contains:

__tui_0 := app.MountPersistent(, 0, func() tui.Component {
	return tui.NewTextArea(
		tui.WithTextAreaPlaceholder("Type here..."),
	)
})

The Go compiler then reports expected operand, found ',' because the first argument to MountPersistent (the parent Component) is missing.

Root Cause

1. generateComponent() at internal/tuigen/generator_component.go:22 resets g.currentReceiver = "".
2. generateFunctionComponent() never sets currentReceiver so it remains empty.
3. When generateComponentElementWithRefs() at internal/tuigen/generator_element.go:464 emits app.MountPersistent(g.currentReceiver, ...), the empty string produces MountPersistent(, 0, ...) — a syntax error.

The same issue exists in generateStructMount() at internal/tuigen/generator_children.go:233 for @Component() calls inside pure components.

Affected Elements

All component elements as defined in isComponentElement() at internal/tuigen/generator_element.go:17-19:

• <textarea />
• <input />
• <modal />
• <markdown />

Expected Behavior

Either:

1. Emit nil as the parent argument for pure components (if MountPersistent accepts nil), or
2. Report a clear compile-time error telling the user that component elements require a receiver (struct components).

Environment

• go-tui (version/commit: current main)