Wire flashcard CSV/JSON import/export and fix theme/responsive gaps

Import and export were stubbed with status-text placeholders; wire them
to the existing backend endpoints with a real file picker (web JS
bridge or native FileDialog) and browser/desktop download, plus an
import preview with create/update/skip/error counts before commit.

Also fix the library and study screens' theme-refresh handlers, which
were no-op stubs so cards never restyled on a theme toggle, and add
responsive layout handling for narrow viewports.

scripts/ui/flashcards_study.gd

@@ -33,12 +33,15 @@ var grade_row: HBoxContainer
 var grade_buttons: Dictionary = {}
 var undo_button: Button
 var empty_label: Label
+var compact_layout := false
 
 
 func _ready() -> void:
 	set_anchors_and_offsets_preset(Control.PRESET_FULL_RECT)
 	add_theme_constant_override("separation", 14)
 	_build()
+	resized.connect(_update_responsive_layout)
+	call_deferred("_update_responsive_layout")
 	FlashcardService.next_card_loaded.connect(_on_next_card_loaded)
 	FlashcardService.next_card_failed.connect(_on_next_card_failed)
 	FlashcardService.review_synced.connect(_on_review_synced)
@@ -98,6 +101,7 @@ func _build() -> void:
 
 	card_panel = PanelContainer.new()
 	card_panel.custom_minimum_size = Vector2(640, 360)
+	card_panel.add_theme_stylebox_override("panel", Brand.content_card_box(UIThemeService.is_dark_mode(), true, Brand.ROYAL_BLUE))
 	center.add_child(card_panel)
 	var card_content := VBoxContainer.new()
 	card_content.add_theme_constant_override("separation", 18)
@@ -155,7 +159,20 @@ func _build() -> void:
 
 
 func refresh_theme_controls() -> void:
-	pass
+	if card_panel == null:
+		return
+	var dark := UIThemeService.is_dark_mode()
+	card_panel.add_theme_stylebox_override("panel", Brand.content_card_box(dark, true, Brand.ROYAL_BLUE))
+	empty_label.add_theme_color_override("font_color", Brand.muted_color(dark))
+
+
+func _update_responsive_layout() -> void:
+	if card_panel == null:
+		return
+	compact_layout = size.x < 720.0
+	var available := maxf(size.x - 48.0, 240.0)
+	card_panel.custom_minimum_size.x = minf(640.0, available)
+	card_panel.custom_minimum_size.y = 300.0 if compact_layout else 360.0
 
 
 func _load_next() -> void: