fix(gui): bind the root <Configure> handler once, not per rebuild

[donislawdev]

What and why

Follow-up to #40. That PR's changelog flagged one thing as deliberately deferred: App._build_ui re-binds <Configure> on the root every rebuild and never removes the old one, so the handler multiplies. This closes it — and a related leak the earlier note missed.

_build_ui runs on every language switch, and the root window outlives it. Binding <Configure> on the root inside _build_ui (with add="+", nothing removing it) piled up one handler per rebuild.

Measured on the fake tkinter (<Configure> handlers on the root):

	after first build	after 3 rebuilds
before	2	8
after	1	1

The #40 note named only _on_root_configure and undercounted (PROJECT_NOTES rule 5): the two banners built with wrapping_label(root, ...) — engine_warning always, admin_warning when non-admin — each bound their own <Configure> on the same persistent root and multiplied identically, half the handlers. A wrapping_label on a short-lived container does not leak (its binding dies with the container); only the two whose container is the root do.

Each handler is cheap, and after #40 the dead ones are no-ops rather than crash records — so this is O(rebuilds) duplicated work on every resize, not a crash. Invisible, but real.

Changes (app.py only)

• _on_root_configure is now bound once in __init__, before the first _build_ui; the line is gone from _build_ui. It reaches its widgets through self, so a single binding always drives the freshly rebuilt ones — no unbind needed, and none was safe (Misc.unbind(seq, funcid) still clears the whole sequence on the oldest Python in the CI matrix, and the root carries other <Configure> bindings).
• The two banners are now plain ttk.Labels wrapped by that same single handler, not wrapping_label. Same look (left/anchored, an initial wraplength refined on the first resize). gui/labels.py and its short-lived-container callers are untouched; wrapping_label is no longer imported by gui/app.py.

Reviewer notes

• New test verified by mutation (rule 5): test_the_ui_rebuild_does_not_pile_up_configure_handlers_on_the_root asserts exactly one <Configure> handler on the root after several rebuilds, and that the banner still wraps to the width-derived value (not just its build-time default — which would pass even with the fold gone). Reintroducing the per-rebuild bind, and dropping the banner wrap, each turns it red.
• No user-facing changelog line, on purpose: identical look and wrapping, no behaviour change. A CHANGELOG.md line for an imperceptible change would be the filler PROJECT_NOTES rule 4 forbids. Documented in CHANGELOG-INTERNAL.md, which also corrects the stale "not fixed here" bullet from fix(gui): stop the UI rebuild from destroying the registry's windows #40.

Checklist

• python -m pytest tests passes locally. (660 passed; run elevated, no admin-only deselects needed.)
• New behaviour has tests (see tests/ for the style). One added, mutation-verified (both halves).
• UI text goes through i18n keys, with both lang/en.json and lang/pl.json updated. (No new UI text — no lang/ edits needed.)
• User-facing changes noted in CHANGELOG.md; technical ones and new tests in CHANGELOG-INTERNAL.md, under [Unreleased]. (Internal-only: nothing a user notices, see reviewer notes.)
• Commits follow Conventional Commits (type(scope): summary).
• No version bump — the owner closes a version via VERSION.txt.

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