#2214: overhaul language selection logic

[samuelkos17]

This PR fixes #2214

Implemented changes:

• The reloaded main view now receives the previous MainController and restores the selected project and workspace (plus the enabled state of the workspace combo and the IDE open buttons) after a language switch.
• The previous selection is only reapplied if the project still exists after the reload; a removed project is skipped gracefully instead of pointing to a stale entry.
• Added a regression test (AppBaseTest#testLanguageSelectionDoesNotResetGuiState) that drives the locale-change reload the same way the GUI does and verifies the GUI state survives, including the stale-project case.

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Testing instructions

Please add conscise, understandable instructions on how a reviewer can test/verify the functionality of your contribution here:

1. Run the build-local-dev.sh script.
2. Open a new shell and run ide gui
3. Inside the GUI, select a project and a workspace, then change the language. The state of the GUI should be kept.

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Checklist for this PR

Make sure everything is checked before merging this PR. For further info please also see
our DoD.

• When running mvn clean test locally all tests pass and build is successful
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  with internal
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• You have formulated clear instructions on how to test your contribution under "Testing instructions"

[coveralls]

Coverage Report for CI Build 32379718556

Coverage increased (+0.04%) to 72.984%

Details

• Coverage increased (+0.04%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 64 coverage regressions across 3 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

64 previously-covered lines in 3 files lost coverage.

File	Lines Losing Coverage	Coverage
com/devonfw/ide/gui/MainController.java	34	73.24%
com/devonfw/ide/gui/App.java	29	0.0%
com/devonfw/tools/ide/tool/ide/IdeToolCommandlet.java	1	78.69%

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Coverage Stats

Relevant Lines:	17586
Covered Lines:	13384
Line Coverage:	76.11%
Relevant Branches:	7777
Covered Branches:	5127
Branch Coverage:	65.93%
Branches in Coverage %:	Yes
Coverage Strength:	3.23 hits per line

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💛 - Coveralls