Vendor backend and frontend into the monorepo

The submodule collapse was only half applied: .gitmodules was deleted but
backend/ and frontend/ were still recorded as gitlinks, so none of their
files were tracked. Replace the gitlinks with the real trees.

Also untrack .env (it carried placeholder secrets) in favour of a tracked
.env.example, drop the committed __pycache__, and narrow the blanket *.png
ignore that would have swallowed design assets.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

NOTEPAD.md

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+This file captures tasks which derive from roadmap tasks (unfinished, deferred), bugs encountered during usage and feature requests/improvements which are not fitting directly in the roadmap.
+Always sort by priority and put tasks blocked by other tasks/features at the dedicated section.
+When working on a task, check for other, similar tasks that could be resolved on the way.
+Use following pattern to classify tasks: TYPE/SCOPE
+Where TYPE could be BUG, FEAT, PERF, CHORE and SCOPE could be UX, UI, AI, SIM, CAD appended by MOBILE if only for mobile use case.
+Don't write temporary reasons for deferring a task in the task description (only strategical reasons should be noted).
+Deferring because out of scope is fine, but don't mention deferring than.