UI changes (so the new schema fields are actually editable, in chronological
order Start → Due/Target):
- New StartDateEditor.svelte (mirrors DueDateEditor.svelte for startDate)
- ControlPanel: render Start Date row above Due Date row in the issue
side panel; both always-visible (no `!== null` guard) so users can set
them on issues that don't have a date yet
- NewMilestone form: Start Date input above Target Date input
- Milestone list view: Start Date column before Target Date column
- TIssueRelation: tighten interface to `extends AttachedDoc<Issue, 'relations'>`
so attachedTo + collection are statically typed. The model class
re-declares `collection: 'relations'` to match the narrower base.
- Drop 4 unused Dependency-kind IntlString keys (FinishToFinish,
FinishToStart, StartToFinish, StartToStart) — they had no consumer
in PR 1; will be re-introduced in PR 4 (dependency editor).
- Simplify migration.ts comments — drop ageing line-references.
Signed-off-by: Michael Uray <michaeluray@users.noreply.github.com>