feat: default UI language to browser language when available

[gantoine]

Description
^{Explain the changes or enhancements you are proposing with this pull request.}

On first visit RomM always started in English (en_US), ignoring the browser's preferred language even when a matching locale was available. This PR makes the app default to the browser language when it maps to one of the shipped locales.

Precedence on startup is now:

1. Saved preference — localStorage["settings.locale"] (or a logged-in user's ui_settings.locale). Unchanged, still wins.
2. Browser language (new) — navigator.languages matched against the available locales.
3. en_US — final fallback when nothing matches.

Matching tries an exact match first (en-US -> en_US, normalizing the separator and casing) then a language-only match (fr -> fr_FR), walking navigator.languages in preference order. A returning user's explicit choice is never overridden. This applies to both v1 and v2 since they share the same RomM.vue bootstrap and language store.

Fixes #3139

Checklist
^{Please check all that apply.}

• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes

Screenshots (if applicable)

N/A — behavioral change only.

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AI assistance disclosure: This change was implemented with AI assistance (PostHog Code / Claude). The browser-language detection logic, the wiring in RomM.vue, and this PR description were AI-generated, then reviewed by a human. Verified locally with npm run typecheck and trunk check.

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Created with PostHog Code

[Copilot]

Pull request overview

Updates the frontend bootstrap so RomM defaults its UI language to the user’s browser preferences when no saved locale is present, improving first-visit UX while keeping the existing saved-preference precedence.

Changes:

• Added browser-language detection/matching logic to the language store.
• Switched initial locale selection in RomM.vue to use the new browser-language detection when no saved locale matches.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
frontend/src/stores/language.ts	Adds detectBrowserLanguage() to map navigator.languages to a shipped locale, with fallback behavior.
frontend/src/RomM.vue	Uses detectBrowserLanguage() during app bootstrap when no stored locale matches.

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