Locale picker navigates to 404 when the translated page lives at a different path #176

Summary

The default theme's locale picker builds the target URL by swapping the locale prefix and keeping the rest of the path. If the translated page lives at a different path, you land on the target locale's 404.

Repro (production, solidbase.dev)

1. Open https://solidbase.dev/guide/features/markdown
2. Open the locale picker ("change locale") → Français
3. You're sent to /fr/guide/features/markdown, which renders the 404 page.

The French translation of that page exists at https://solidbase.dev/fr/guide/markdown.

I checked this with Playwright (Chromium). The same happens on the Solid 2 port (#174), because the logic is unchanged there.

Expected vs actual

• Expected: switching locale goes to the translated page, or at least to a page that exists in that locale.
• Actual: you land on /<locale>/<same rest path> even when no page exists there, and get a 404.

Cause

setLocale in src/client/locale.ts (on main and solid2) calls getSolidBaseRoutePathWithRest(routes, { ...current, [LOCALE_AXIS]: locale.option.name }, currentRouteMatch()?.restPath ?? "/") and navigates to the result. getSolidBaseRoutePathWithRest (src/config/route-config.ts) just joins the locale route prefix with the current rest path. It never checks whether that route exists.

Possible fixes

• Fallback: if the computed target isn't a known route, go to the nearest existing ancestor in the target locale (e.g. /fr/guide), or to the locale's index.
• Explicit mapping: let pages declare their counterparts, e.g. a frontmatter key such as translations: { fr: "/fr/guide/markdown" } or a config map. Use it before the prefix swap.
• Both: use the mapping when present, otherwise fall back.

— Claude (Opus 5.5), posting on behalf of @devagrawal09