UI/UX pass: readability, feedback, safer actions, DRK branding

Items 1–12 of the interface review:

- Larger UI font (UI_FONT_SIZE) across widgets, menus, dialogs, Treeview rows.
- DRK-red header bar; window title shows entry count; runtime-drawn red-cross
  icon (also written to data/icon.png for the launcher).
- Status bar colour-coded by severity; transient messages auto-clear after 8 s,
  queue-count messages stay.
- Tab 1: more form spacing, digits-only PLZ, explicit "Auswahl entfernen" +
  Delete key for queue rows, confirm on "Warteschlange leeren", modal on
  duplicate save.
- Tab 2: raw lat/lon columns replaced by one "Karte" column (checkmark / red
  "fehlt"); ▲/▼ arrow on the active sort column; hint line always shows the
  saved-entry count.
- "Karte öffnen" with no coordinates shows a dialog, not just a status line.
- Window size/position remembered between sessions (data/window.json).

Verified by building and driving the window under Homebrew Python 3.12 / Tk 9;
non-GUI smoke tests still pass. Docs updated (changelog, overview, improvements,
data-model, setup, dev-notes).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/setup.md

@@ -73,14 +73,15 @@ Type=Application
 Name=DRK Blutspende – Arbeitsorte
 Comment=Einsatzorte protokollieren und auf der Karte anzeigen
 Exec=/home/USER/Apps/DRK_Blutspende_Orte/run.sh
-Icon=/home/USER/Apps/DRK_Blutspende_Orte/icon.png
+Icon=/home/USER/Apps/DRK_Blutspende_Orte/data/icon.png
 Terminal=false
 Categories=Utility;
 ```
 
-Replace `USER` with the real username. Add any PNG as `icon.png` (the DRK logo
-works well). The entry then shows up in the Mint menu and can be pinned to the
-panel or the desktop.
+Replace `USER` with the real username. The app writes a simple red-cross
+`data/icon.png` on first run; drop in the real DRK logo at that path if you have
+one. The entry then shows up in the Mint menu and can be pinned to the panel or
+the desktop.
 
 ## Python / Tk version