Feat/frontend locale config

[AndreRFerreira]

Summary:
Add configuration-driven locale resolution to support server-published locale policies, locale fallback chains, and restricted locale lists for multi-language deployments. Closes #1606

Description for the changelog:
Configuration-driven locale resolution: client reads locale policy from server configuration, resolves user locale against allowed locales with fallback chain, and surfaces configuration load errors in the UI

Declaration:
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Other info:

• Locale resolver (src/service/locale/locale-resolver.js): Pure-function service that resolves user locale against a server-defined allowed-locale list with i18next-style fallback chain. Handles browser navigator.languages, server defaultLocale, and SUPPORTED_LOCALES intersection.
• HTTP client timeout (src/service/api/api.js): Adds configurable default request timeout to prevent hung requests blocking locale resolution (spinners stay forever).
• Config store module (src/store/modules/config.js): Sanitizes server config (validates booleans/strings against SUPPORTED_LOCALES), exposes allowedLocales/defaultLocale getters, and surfaces configError state for error handling.
• Locale store module (src/store/modules/locale.js): Wires the resolver into Vuex — resolves locale on RESOLVE_LOCALE dispatch, selects locale on LOCALE_SELECTED, syncs i18n instance, and enforces the allowed-locale guard.
• HomePage UI (src/views/HomePage.vue): Adds loading spinner while config is fetched, shows toast on config load failure, and extracts resolveProviders() as a pure function for testability.
• LocaleSelect component: Wires locale resolution into dropdown selection.
• i18n constants (src/i18n/index.js): Extracts DEFAULT_LOCALE, SUPPORTED_LOCALES, and installLegacyCompat() for reuse across modules.
• Config load error translation (src/i18n/*.js): Adds home.errors.configLoadFailed key to all locale files.
• E2E tests: Config API mock server, HomePage config-driven tests, locale section tests.
• Unit tests: Full coverage for locale resolver, config store, locale store, config actions, locale actions, and i18n constants.

[AndreRFerreira]

Unrelated test fix coming up next: resetDesktopApp state pollution in desktop E2E menu tests

Problem: The 'opens a new model editor through File > New Model' test was failing intermittently with URL app://./index.html#/desktop/Saved%20As%20From%20Menu/edit instead of the expected app://./index.html#/desktop/New%20Threat%20Model/edit. The URL contained the title from a previous test (SAVED_AS_MODEL_TITLE), indicating cross-test state leakage.

Root cause: resetDesktopApp() cleared sessionStorage before browser.refresh(). Between the clear and the refresh, window.location.hash = '#/' triggered Vue Router navigation, causing Vuex mutations to fire. The vuex-persist plugin then re-wrote the stale threat model state back to sessionStorage. When the page reloaded, Vuex re-hydrated from the re-persisted stale data.

The Fix: Clear session/localStorage after the first page refresh, then do a second refresh so the app reinitializes from truly empty storage. (td.vue/tests/e2e/desktop/support/helpers.js)

Not related to my feature work — this is a pre-existing test isolation bug in the desktop E2E test infrastructure.

[AndreRFerreira]

Thanks for the support @lreading I've never used this before, so I’ve to lean on you here as I'll have to read about it rather than just using.
On the spinner (and both questions), sounds about right, I’ll refactor today, probably add/use a "isElectron" flag, then the issue will go away. I missed that.
Please do let me know on the review of other pieces where I’ve missed anything or for potential improvements.
EDIT: I see a conflict with .gitignore i'll address that later today also
EDIT: I see the issue with the e2e, adding a fixes for this also, hope it's the ones you'r are aware of

[AndreRFerreira]

I've taken some liberties in applying other bits and pieces, sorry for the larger review requirement