[SPPM-310] Add milestones to the project lifecycle widget

[HDinger]

Ticket

https://community.openproject.org/wp/SPPM-310

What are you trying to accomplish?

• The "Project timeline" widget on the overview is extended to show all milestones of that project
  • The milestone objects:
    • are linked (to the full view of the Milestone)
    • are shown below the project phases
    • show a tooltip on hover similar to the project phases but are extended by a link to the milestone full view
    • are not shown if the user is lacking permissions
  • If there are two milestones on the same day, we show them above each other
• The timeline widget is changed to:
  • be hidden when the user has neither permission to view project lifecycle attributes nor work packages
  • show a Blankslate when there are neither project lifecycle attributes nor milestones
• In the footer of the widget, there is a link to the Gantt view of all milestones called "Milestones Gantt"Name tbd

Screenshots

[github-actions]

Deploying openproject with ⚡ PullPreview

Field	Value
Latest commit	2a19574
Job	deploy
Status	✅ Deploy successful
Preview URL	https://pr-24237-sppm-310-add-miles-ip-167-233-23-8.my.opf.run:443

View logs

[github-actions]

Caution

The provided work package version does not match the core version

Details:

• Work package URL: https://community.openproject.org/wp/SPPM-310
• Work package version: 17.8.0
• Core version: 17.7.0

Please make sure that:

• The work package version OR your pull request target branch is correct

[github-actions]

Warning

Flaky specs

• rspec ./modules/backlogs/spec/features/inbox_column_spec.rb[1:7:1]

🤖 Ask Copilot to investigate

Copy the prompt below into a new comment on this PR to delegate the investigation to GitHub Copilot. It will look into the flakiness and open a separate pull request with you as reviewer.

@copilot The following spec(s) are flaky in CI (first seen on PR #24237, linked for reference only):

- `rspec ./modules/backlogs/spec/features/inbox_column_spec.rb[1:7:1]`

Treat this as a standalone task, unrelated to PR #24237. Create a new branch from origin/dev and open a new pull request targeting dev — do not stack it on PR #24237 or reuse that branch.

Follow the playbook in docs/development/testing/handling-flaky-tests/README.md to find the root cause and fix the underlying race — do not skip, delete, or weaken the spec to make it pass; disabling is a last resort per the playbook, and only with a bug ticket. Verify the fix by running the spec(s) repeatedly (e.g. `script/bulk_run_rspec --run-count 10`).

If you cannot reproduce the flake or are not confident in a fix after reasonable investigation, do not fabricate a change or skip the spec to force CI green. Instead, leave the pull request in draft and document what you tried, the suspected cause, and any leads in its description, then assign @HDinger to take over.

Once the fix is verified, title the PR after the spec(s) it fixes, and use the PR description to explain the root cause, how the change resolves it, and the before/after results. Label the PR `flaky-spec`, assign @HDinger, and request a review from @HDinger.
On every commit, set @HDinger as the sole co-author with a `Co-authored-by:` trailer (use their GitHub no-reply email so it links to their account), so it is traceable who dispatched the fix.

[bsatarnejad]

Since the widget can now render with only view_work_packages, should this return [] unless the user also has view_project_phases? Otherwise the milestone-only permission path still sends phase data to the frontend.