Split install for non-admin users

The target laptop account has no sudo. install.sh now runs without root once the
system packages exist; if they're missing and the account can't sudo it prints
the exact command and stops. New install-system.sh holds the one-time
"sudo apt install python3-tk python3-venv git" for an admin account.

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

docs/setup.md

@@ -3,7 +3,24 @@
 Target machine: **Linux Mint laptop**, single non-technical user. Day-to-day use
 is a **double-click from the menu**; the terminal is needed once, for install.
 
-## Quick install (Linux Mint)
+## Install (Linux Mint)
+
+The target user's account is a **Standard** account (no sudo). Install is
+therefore two steps: one system step by an admin, then the rest by the user.
+
+### Step 1 — system packages (admin account, once)
+
+`python3-tk`, `python3-venv` and `git` are system packages. From an account with
+admin rights (Patrick's):
+
+```bash
+cd /path/to/DRK_Blutspende_Orte
+./install-system.sh          # sudo apt install python3-tk python3-venv git
+```
+
+Skip this if those three are already present.
+
+### Step 2 — the app (user account, no sudo)
 
 ```bash
 mkdir -p ~/Apps && cd ~/Apps
@@ -12,10 +29,11 @@ cd DRK_Blutspende_Orte
 ./install.sh
 ```
 
-`install.sh` does everything:
+`install.sh` needs no sudo once step 1 is done. It:
 
-1. `sudo apt install python3-tk python3-venv python3-pip git` (asks for the
-   password),
+1. checks the system packages are there (if not, and the account *does* have
+   sudo, it installs them; otherwise it prints the exact `install-system.sh`
+   line an admin must run and stops),
 2. creates `.venv` and installs the pinned `requirements.txt`,
 3. writes `~/.local/share/applications/drk-blutspende-orte.desktop` with the
    correct absolute paths and the bundled `icon.png`.
@@ -24,13 +42,19 @@ After it finishes, **"DRK Blutspende – Arbeitsorte"** is in the application me
 (log out/in once if it doesn't show immediately). First manual test: `./run.sh`.
 
 > Clone it — don't download a zip. The in-app updater and `run.sh` both rely on
-> it being a real git checkout.
+> it being a real git checkout. The clone lives in the **user's** home so she can
+> update it without sudo.
+
+If you do both steps yourself in one go: run `./install-system.sh` as admin,
+then `su - <heraccount> -c '~/Apps/DRK_Blutspende_Orte/install.sh'` (or just log
+in as her and run step 2).
 
 ## What the pieces are
 
 | File | Purpose |
 |------|---------|
-| `install.sh` | one-time setup (above). Safe to re-run. |
+| `install-system.sh` | admin, once: `apt install python3-tk python3-venv git`. |
+| `install.sh` | user, no sudo: venv + deps + menu entry. Safe to re-run. |
 | `run.sh` | launches the app from `.venv`; the `.desktop` entry calls this. |
 | `icon.png` | bundled launcher/window icon (red cross). Replace with the real DRK logo if wanted. |
 | `requirements.txt` | fully pinned, tested on Python 3.12. |