refactor(file-upload): SSR-first state, click-to-replace, cleaner preview

- Server renders initial file-upload state (src, alt, label text,
  has-current class) so there is no flash of broken-image icon while
  JS boots. JS no longer duplicates that work — it only reacts to
  user input (select, clear, drag-and-drop) and uses data-current-src
  solely to restore the server file after a blob preview.
- Clicking the current preview image now opens the file dialog, mirrors
  the Replace button, with hover affordance (cursor + primary border).
- Drop the transparency checker pattern in the preview — the admin UI
  does not need Figma-style transparency semantics. Preview is now a
  flat theme-aware surface (--app-preview-bg) that keeps white or
  black logos visible against a neutral gray.
- Move tenant favicon out of the aside into the Master-data tab as its
  own details block next to the Tenant-logos section. Uses the same
  barrier-form pattern (HTML5 form attribute + app-file-upload.phtml)
  for a consistent instant-upload UX.
- Tenant-logo slot label is a native <label> element — picks up the
  global form-label typography and spacing, no custom muted-color
  class needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/css/base/variables.base.css

@@ -120,8 +120,7 @@
   --app-text-selection-color: rgba(2, 154, 232, 0.25);
   --app-muted-color: #646b79;
   --app-muted-border-color: rgb(231, 234, 239.5);
-  --app-preview-checker-bg: #ffffff;
-  --app-preview-checker-fg: #eef0f3;
+  --app-preview-bg: #e8ebef;
   --app-button-filled-shadow:
     inset 0 1px 0 rgba(255, 255, 255, 0.2),
     0 1px 2px rgba(0, 0, 0, 0.12);
@@ -632,8 +631,7 @@
   --app-text-selection-color: rgba(1, 170, 255, 0.1875);
   --app-muted-color: #6e6e6c;
   --app-muted-border-color: #2f2f2d;
-  --app-preview-checker-bg: #2a2a28;
-  --app-preview-checker-fg: #363634;
+  --app-preview-bg: #262a2f;
   --app-button-neutral-bg: #30343b;
   --app-button-neutral-bg-hover: #3b4048;
   --app-button-neutral-border: #464b54;