Add reusable UI components for Phase 7 UI Polish

- Introduced `EmojiQuickGrid`, `FontPresetPicker`, and `PresetSizeToggle` classes for emoji selection, font presets, and size toggles respectively.
- Implemented configuration methods for each component to handle dynamic content and theming.
- Created tests for the new components to ensure functionality and integration with the existing UI.
- Added a comprehensive architecture specification for Phase 7 UI Polish detailing scope, implementation, and validation strategies.
- Updated existing theme handling to support new UI elements and ensure consistent styling across the application.

tests/test_phase7_theme.gd

@@ -0,0 +1,22 @@
+extends SceneTree
+
+const THEME_SCRIPT := preload("res://scripts/sandbox_theme.gd")
+var _failures: int = 0
+
+func _init() -> void:
+	var theme = THEME_SCRIPT.new()
+	_check(theme.load_from(), "committed v2 theme loads")
+	_check(theme.integer("fonts.sizes.tooltip", 0) == 18, "v2 font sizes resolve")
+	_check(theme.color("mode_colors.edit", Color.BLACK) == Color("#22c6ff"), "mode color resolves")
+	var first: StyleBoxFlat = theme.stylebox("palette")
+	first.bg_color = Color.RED
+	var second: StyleBoxFlat = theme.stylebox("palette")
+	_check(second.bg_color != Color.RED, "stylebox callers receive isolated copies")
+	_check(not theme.swatches().is_empty() and not theme.size_presets().is_empty(), "widget defaults available")
+	print("PASS" if _failures == 0 else "FAIL: %d" % _failures)
+	quit(0 if _failures == 0 else 1)
+
+func _check(condition: bool, message: String) -> void:
+	if not condition:
+		_failures += 1
+		push_error("Phase 7 theme: " + message)