[3.0] Give these captions to the fields they belong to

[albertlast]

Description

Nine form controls carry a caption that is drawn beside them and never associated with them. A screen reader announces the field with no name, and clicking the caption does nothing.

The theme already does this correctly elsewhere, which is what makes these read as oversights rather than a decision:

• the same subject caption is a label bound to its input in the posting form and the newsletter form, and a bare <span> in the personal message and mailing forms;
• the group form binds the "Required posts" caption on the page that edits a group, and leaves it unbound on the page that adds one.

Fixed the way the working ones already do it:

template	fields
PersonalMessage	To, Bcc, Subject
ManageMail	Subject
Memberlist	Search for — it had a <label> with no for at all
ManageMembergroups	Required posts, on the add page
Reminder	Username/Email — the form's only field
Profile	Primary Membergroup, Signature

Where the input already had an id the label simply names it; where it did not, it gets one. No user-facing text is added or changed — every caption here is already on the page.

How they were found

By walking 484 controls over 43 pages and asking of each whether anything gives it an accessible name: a label[for], a wrapping label, aria-label, aria-labelledby, title or placeholder.

pages loaded:            43
controls checked:       484
caption present but unlinked:  26

These nine are the subset where that caption belongs to exactly one control and to nothing else.

Deliberately not touched

• The ban form. Its captions are already labels, and they correctly name the checkbox that arms each trigger rather than the text field beside it. Pointing them at the text field instead would take the name off the checkbox.
• Three places where one caption heads several controls — the search form's message-age range, the group form's permission selects, the avatar block. Those want a fieldset and legend, not a label.

Both need new wording, so they are their own change. The remaining 153 controls with no visible caption at all — the header search box among them — likewise need new translator-facing strings and are not in scope here.

Nothing moves

Computed colour, font, display and box geometry of every dt, dd and named control on the six affected pages, plus the height of each page, before and after:

stable records compared: 177
DIFFERENCES: 0

(The raw comparison first reported differences that were entirely SMF's per-request session-token field name and the autosuggest's dummy_NNNNNN inputs, which are random on every page load. Excluding those, the six pages are identical, page heights included.)

Part of wave 7 of the #7933 split, and a continuation of #9500 and #9549.

Issues References (Fixes|Related|Closes)

Related #9500