fix(ui): make the module tab strip a scrollable single column

[DeepDiver1975]

Summary

Reported live, with a screenshot on a "relative small screen": the left module tab strip
(AUFBAU, ETB, ..., WASSERFÖRDERUNG, ABBAU) became unusable — WASSERFÖRDERUNG rendered directly
beside AUFBAU, ABBAU beside ETB, both crammed and unreadable.

Root cause: TabControl's TabStripPlacement="Left" (the "dispatch sidebar" rail in
Theme/Styles.axaml) relied on Avalonia's default items panel for that placement — a
vertically-oriented WrapPanel. Once the rail ran out of height for all 11 fixed-MinHeight-50
TabItems, it wrapped into a second column starting back at the top, exactly matching the
screenshot.

Forcing a single-column panel alone wasn't sufficient — confirmed via a headless test that
Avalonia's default template has no ScrollViewer around PART_ItemsPresenter at all, so
without wrapping, the last tabs would simply be clipped off the bottom edge with no way to reach
them (the same class of dead-end as the earlier Wasserförderung map zoom/pan bug fixed in #170,
just in the navigation rail this time).

Fix

Replaced TabControl's template with a small purpose-built one — this app only ever uses
TabStripPlacement="Left", so there's no need to handle other placements generically: a
single-column ItemsPresenter wrapped in its own ScrollViewer, laid out beside the content
host, instead of patching around Avalonia's default template's WrapPanel.

Verification

• New ModuleTabStripLayoutTests.Tabs_stay_in_a_single_column_top_to_bottom_on_a_short_window:
  confirmed red before the fix (reproduced the exact Y-position jump backward that causes the
  visual overlap), green after — all tabs stay in one column (monotonically increasing Y), and
  the last tab (ABBAU) is reachable by scrolling the rail into view at a window height that
  genuinely needs it.
• Full dotnet test LageBuch.sln: 948 tests passing, 1 pre-existing skip. This retemplates a
  control used throughout the whole app's navigation, so the full suite (extensively exercising
  tab switching) is the relevant regression check here, not just the new test.

Screenshot

Please paste /tmp/lagebuch-shots/module-tab-strip-small-screen-fixed.png into the PR body — I
can't upload images directly. It shows the rail scrolled to its bottom at a genuinely short
window: ATEMSCHUTZ through ABBAU all render single-column and legible, with a visible scrollbar
on the rail itself.

Checklist

• Commits follow Conventional Commits and are DCO signed off (git commit -s)
• dotnet build and dotnet test pass locally (948 tests, 1 pre-existing skip)
• UI changes include before/after screenshots
• No real master data / personnel data committed (screenshot uses fictional data)

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

https://claude.ai/code/session_01UwN31QccH98YV9eEc2bue2

[DeepDiver1975]

Closing as redundant: this branch stacks on feat/wasserfoerderung-plan-a (#152), which forked from main before #151 merged — so this independently rediscovered and re-fixed the exact same left-rail wrapping bug #151 already solved on main (wrapping PART_ItemsPresenter in an unbounded-height ScrollViewer instead of replacing the WrapPanel). #151's fix is the one that should stand; this one is being dropped rather than reconciled. The whole Wasserförderung stack (#152 ← #164 ← #169 ← #170 ← this) is being rebased onto current main to pick up #151 and everything else merged since.