fix(locale-switcher): make flag the visual anchor (32x24, no border, no lazy load)

Martin feedback round 3: dropdown still looked 'fuerchterlich' even
with the official flag-icons. Root cause: 14px vertical padding
around an 18px-tall flag meant the flag occupied only 39% of the
trigger height and was dwarfed by whitespace. Plus a 1px black
box-shadow border made flags look 'boxy', and loading='lazy' caused
empty boxes on the four menu flags the moment the <details> opened.

Changes:
- Flag size 24x18 -> 32x24 (+78% area, ~4:3 matches flag-icons)
- Trigger padding 14px 8px -> 6px (flag now 73% of trigger width,
  55% of trigger height, was 46%/39%)
- Drop the artificial 1px black box-shadow outline on flags
- Drop border-radius on flags (real flag-icons look better as
  crisp rectangles)
- Drop object-fit: cover (no longer needed for SVG)
- Drop loading='lazy' and decoding='async' (4 small SVGs, must
  be ready the moment <details> opens, not flash empty boxes)
- min-height: 44px restored on trigger for WCAG 2.5.5 touch target
- Menu border-radius 8 -> 10px, padding tightened, font-size 0.85
  -> 0.9rem for label legibility
- Two-layer box-shadow on menu for subtle elevation

tests/Controllers/LocaleSwitcherTest.php

@@ -111,7 +111,7 @@ final class LocaleSwitcherTest extends TestCase
             $img = LocaleSwitcher::flagImg($code);
             self::assertStringStartsWith('<img', $img);
             self::assertStringContainsString('class="flag"', $img);
-            self::assertStringContainsString('width="24" height="18"', $img);
+            self::assertStringContainsString('width="32" height="24"', $img);
             self::assertStringContainsString('alt=""', $img);
             self::assertStringEndsWith('>', $img);
         }