feat(tracker): add Gantt scheduling schema (startDate + IssueRelation)

Schema-only foundation for the upcoming Gantt-chart view in tracker.
No UI in this PR.

Changes:
- Issue.startDate: Timestamp | null (interface + IssueDraft + @Prop with @Index)
- Milestone.startDate: Timestamp | null (interface + @Prop, reusing the
  existing tracker.string.StartDate IntlString)
- New DependencyKind type ('finish-to-start' | 'start-to-start' |
  'finish-to-finish' | 'start-to-finish')
- New IssueRelation AttachedDoc class with kind: DependencyKind, signed
  lag: number — registered in models/tracker via TIssueRelation
- 7 new IntlString keys: IssueStartDate, GanttDependency,
  GanttDependency{FinishToStart,StartToStart,FinishToFinish,StartToFinish},
  GanttLag — all 13 locales updated
- Cross-plugin literal updates in importer + github sync to satisfy the new
  required Issue.startDate / Milestone.startDate fields:
  - packages/importer/src/importer/importer.ts: AttachedData<Issue> literal
  - services/github/pod-github/src/sync/issueBase.ts: 'startDate' added to
    GithubIssueData Omit list (github sync does not own scheduling)
  - services/github/pod-github/src/sync/issues.ts + pullrequests.ts:
    AttachedData<Issue|GithubPullRequest> literals

Out of scope (deferred to follow-up PRs):
- UI for Gantt view, drag/resize, dependency editor, critical path
- blockedBy → IssueRelation migration (ships atomically with the writer
  redirect in the dependency-UI PR)
- LinkIssues permission (tracker uses forbid-style permissions; needs
  maintainer discussion)
- Activity-feed wiring for IssueRelation (needs a producer to test against)
- IssueTemplate.startDate (template propagation semantics undecided)

Signed-off-by: Michael Uray <michaeluray@users.noreply.github.com>

plugins/tracker-resources/src/components/CreateIssue.svelte

@@ -186,6 +186,7 @@
       priority: priority ?? IssuePriority.NoPriority,
       space: _space as Ref<Project>,
       component: component ?? $activeComponent ?? null,
+      startDate: null,
       dueDate: null,
       attachments: 0,
       estimation: 0,
@@ -312,6 +313,7 @@
         _id: generateId(),
         space: _space as Ref<Project>,
         subIssues: [],
+        startDate: null,
         dueDate: null,
         labels:
           p.labels !== undefined
@@ -488,6 +490,7 @@
         rank: '',
         comments: 0,
         subIssues: 0,
+        startDate: object.startDate,
         dueDate: object.dueDate,
         parents:
           parentIssue != null

plugins/tracker-resources/src/components/SubIssues.svelte

@@ -86,6 +86,7 @@
         rank: '',
         comments: 0,
         subIssues: 0,
+        startDate: subIssue.startDate ?? null,
         dueDate: null,
         parents,
         reportedTime: 0,

plugins/tracker-resources/src/components/milestones/NewMilestone.svelte

@@ -34,6 +34,7 @@
     status: MilestoneStatus.Planned,
     comments: 0,
     attachments: 0,
+    startDate: null,
     targetDate: Date.now() + 14 * 24 * 60 * 60 * 1000
   }
 

plugins/tracker-resources/src/components/templates/DraftIssueChildEditor.svelte

@@ -71,6 +71,7 @@
       assignee: project.defaultAssignee ?? null,
       status: project.defaultIssueStatus,
       space: project._id,
+      startDate: null,
       dueDate: null,
       subIssues: [],
       attachments: 0,