frontend: Localize startup failures with typed errors

[rwinkelman]

Description

Startup failures from OBSApp::AppInit and GetLocaleNames were thrown as bare const char * strings. The catch site in obs-main.cpp could only surface that English text, so users on other locales saw an untranslated error dialog.

This adds OBSInitException with an OBSInitErrorCode, keeps a short developer detail string for logging, and maps each code to a new Init.Error.* string in en-US.ini via QTStr for the dialog. Other const char * throws (outputs/modules) are left alone for a later change; the old catch remains as a fallback.

Motivation and Context

Fixes #13394. Matches the typed-error approach discussed on that issue.

How Has This Been Tested?

Built the changed frontend sources locally with the project clang-format config applied. Full OBS CI was pending first-time contributor workflow approval on the earlier push; please re-run after reopen.

Manual check still needed: normal launch, and a forced AppInit failure path to confirm the dialog uses Init.Error.* and the log line includes the error code name.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the Contributing Guidelines.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[rwinkelman]

CI is waiting on first-time contributor workflow approval (action_required). Once Approve and run workflows is clicked I can fix anything that fails. Taking #13394 as discussed on the issue (typed init exception + localized dialog).

[Fenrirthviti]

This does not follow our PR template/contribution guidelines, and smells like AI.

[rwinkelman]

Sorry about the earlier description — I missed the project PR template. Reopened with the template filled out, commits squashed to a single frontend: commit, and clang-format applied. Happy to adjust the design if you'd rather keep init throws as plain strings.