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/changelog.md

@@ -2,10 +2,20 @@
 
 Notable changes to the app. Newest first.
 
+## 2026-09-07 — install split for non-admin users
+
+The target account has no sudo, so install is two scripts:
+
+- `install-system.sh` – admin runs once: `apt install python3-tk python3-venv git`.
+- `install.sh` – user runs, **no sudo**: checks the system packages, creates
+  `.venv` + installs `requirements.txt`, writes the `.desktop` menu entry. If a
+  package is missing and the account can't sudo, it prints the exact admin
+  command and stops.
+
 ## 2026-09-07 — one-command install + launcher
 
-- `install.sh` – one-time Linux Mint setup: apt packages, `.venv` +
-  `requirements.txt`, and a `.desktop` menu entry with correct absolute paths.
+- `install.sh` – Linux Mint setup: `.venv` + `requirements.txt`, and a
+  `.desktop` menu entry with correct absolute paths.
 - `run.sh` – launches the app from `.venv`; the menu entry calls this.
 - `icon.png` – bundled red-cross launcher/window icon (256×256); the app now
   prefers this over the runtime-drawn one.