fix(locale): review and correct Spanish (es-ES) translations

[soymgomez]

Issue: none — translation-only fix, in the spirit of locale/README.md ("Feel free to submit pull requests with improved translations at any time"). Happy to open one if you'd prefer.

Review of the Spanish (es-ES) locale against locale/en. The strings were machine-translated by the locale workflow and a fair number of them picked the wrong sense, broke placeholders, or drifted from the source. 100 strings touched across the two files; nothing outside locale/es-ES/ is modified.

Split into four commits so they can be reviewed (or dropped) independently.

1. Broken placeholders and wrong strings

These are actual bugs, not style:

Key	Problem
email.new_post.text	Said "left a comment on" instead of "created a new post" — copy-paste from email.new_comment.text. The "new post" notification email described the wrong event.
email.footer.subscription_notice3	Dropped the {view} and {change} placeholders entirely, so that email footer rendered with no links at all.
modal.showvotes.message.zeromatches	Used {0} where the source has {query}, so the search term was never interpolated.
validation.custom.invalidcustomdomain	Missing the opening quote: {domain}' no es....
feed.post.footer	Plural units left in English (# vote, # comment).
feed.global.title, feed.post.title	Missing from es-ES altogether. Added.

server.json is also reordered to match locale/en/server.json, which is what puts the two new keys in the right place.

2. Context mistranslations

The machine translation picked the wrong sense. Some examples:

• editor.markdownmode: "Switch to markdown editor" → "Cambiar al editor de rebajas" — markdown as in a price discount. 🙂
• action.postcomment: "Post" (the comment box submit button) → "Correo" (mail).
• action.respond: "Update Status" (button) → "Estado de actualización" ("update's status").
• label.following: the Follow button's active state → "Siguiente" ("next").
• showpost.moderation.declined: "Post declined successfully" → "El puesto fue rechazado" — puesto as in a job position.
• modal.signin.header: "Join the conversation" → "Envíe sus comentarios".
• error.unauthorized.text was a verbatim copy of error.forbidden.text and never mentioned signing in.
• moderation.*.published.verified: "...and user verified" became "...verified by the user", inverting the subject.

Also, three server.json strings still described the old magic-link sign-in (email.signin_email.text, .confirmation, email.signup_email.text); Fider emails a one-time code now.

3. Terminology

"post" was translated four different ways — publicación / entrada / mensaje / puesto. Settled on publicación, already the majority. Plus: pagination.next was "Próximo" rather than "Siguiente"; "All Feedback" was "Todos los comentarios", which collides with actual comments, now "Todas las sugerencias" to match postdetails.backtoall; and some calques ("es requerido", "es inválido", "no está soportado").

4. Register and style

• The file mixed tú (52 strings) and usted (~20). Normalised to tú, the existing majority.
• Removed English Title Case from short labels ("Marcar Todo como Leído", "Política de Privacidad", "Eliminar Mi Cuenta"…) — Spanish capitalises only the first word.
• Removed trailing periods the source strings don't have (mostly moderation toasts).
• "No se aprobó la publicación" / "No rechazó el comentario" asserted the moderator declined it; the source says the action failed. Now "No se pudo aprobar/rechazar…".
• signin.message.emailsent had a typo ("session" for "sesión").
• "¡Caracoles!" is very regional; replaced with a neutral "¡Vaya!".

Verification

Checked programmatically against locale/en:

• both files parse as JSON and the key sets match en/ exactly;
• every {placeholder} and every <0>-style tag has parity with the source string;
• ICU plural/select structures are unchanged.

Checklist

• This PR is linked to a GitHub issue (bug fix or an agreed change). — translation fix; see note above
• New feature only: n/a
• I've read the Contributing guide and the feature policy in discussion #1529.