i18n: extract the task-form picker fields (Part of #145)

[everysingletear]

i18n: extract the task-form picker fields (chores namespace)

Part of #145. This is the narrowed version of this PR, as announced in
#145 and #168.

The original diff covered twelve files; six of them were rewritten under
me since — the modal unification, #186, and the AddTaskModal autocomplete
work. Those hunks no longer applied, which is why GitHub kept showing
this as conflicting. Rather than resolve conflicts that aren't really
conflicts, this force-push reduces the PR to the seven files that are
still untouched on develop:

AssigneePickerField, AttachmentPickerField, DueDatePickerField,
LabelsPickerField, ProjectPickerField, SmartTaskTitleInput, SubTask

The rewritten half goes back in the queue behind your work, and I'll
re-extract it against the current shape rather than replaying the old
diff.

Rebased on current develop. 16 keys added to en/chores.json, 1 to
en/common.json. English only — no translations, no behaviour change;
every t() value is verified to appear character-for-character in the code
it replaces.

[everysingletear]

Apologies for leaving this sitting red — I've now audited it properly rather than just rebasing and hoping.

Six of the twelve files here were rewritten on develop since I branched: DueDatePickerField lost 522 lines to the project-management rework (#186), AddTaskModal gained 233 in the same change, and RepeatPickerField / NotificationPickerField went through the modal-and-button unification. SmartTaskTitleInput picked up the audio-input work. That's roughly 70% of this diff pointing at code that no longer exists — not something a rebase can resolve sensibly, since the strings have to be re-extracted against the new structure anyway.

The other six files are untouched, and they happen to be a coherent set on their own: the task-form field components. So rather than leave this conflicting indefinitely, I'd like to force-push it down to just those and retitle it "i18n: extract task form field strings" — AdvancedOptionsSection, PriorityPickerField, AssigneePickerField, ProjectPickerField, AttachmentPickerField. (SubTask.jsx is clean on develop but sits in an open branch, so I'm leaving it out.)

That turns this into a small, conflict-free, reviewable PR. The rewritten half goes back into the queue behind the modal work, and I'll send it once that's settled.

Full audit of the whole fork — what's ready and what's blocked on which refactor — is in #145. Shout if you'd rather I just close this one instead.

[everysingletear]

Rebased on current develop as well, so this one is no longer conflicting either.

One thing worth flagging since it changed a file rather than just the dictionary: AttachmentPickerField is re-extracted on top of your z-index refactor (462af85) instead of being carried across. My branch predates it and still imported constants/zIndex, which you removed there — carrying my version over would have reintroduced a dead import. The extraction itself is the same two strings.

[meauxt]

@everysingletear this need a rebase also