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/data-model.md

@@ -59,3 +59,14 @@ closed.
 
 Rotating log file (512 KB × 3 generations). Git-ignored. Records saves, map
 generation, geocoding failures, and uncaught exceptions.
+
+## `data/window.json`
+
+`{"geometry": "<w>x<h>+<x>+<y>"}` – the window size and position, saved on close
+and restored on start. Git-ignored, per-machine. Safe to delete (window opens at
+its default size).
+
+## `data/icon.png`
+
+The red-cross window icon, drawn at first run and written here so the `.desktop`
+launcher can point at it. Git-ignored (regenerated). Delete to force a redraw.