Keep project submission status persisted after failed validation

Parent scope: #56 (preserved cohort project behavior)

User outcome

When a learner's first project submission is rejected by validation, the page must not claim
that a submission was saved. The learner sees the validation errors, the entered form remains
available for correction, and the status stays explicitly Not submitted until a submission is
actually persisted. A persisted submission continues to show Submitted with its saved time and
the update action.

This is a narrow target-native correctness slice. It does not complete #56 or change project
validation, scoring, peer review, or cohort migration behavior.

Scope

• Expose an explicit persisted-submission boolean from the project-page context rather than using
  the truthiness of a temporary validation object.
• Use that boolean consistently for the status badge, saved-status text, update/remove affordances,
  and the save/update button label.
• Preserve the existing invalid-submission form values and validation errors.
• Refresh the adoption manifest for every candidate-owned copied file whose bytes change.

Non-goals

• No change to project URL validation or network safety rules.
• No change to persisted submission data, scoring, peer assignment, or notifications.
• No completion claim for the broader #56 project/peer-review contract.

Acceptance criteria

• A rejected first submission with no persisted row renders Not submitted and Status: Not
  saved yet, with no saved timestamp or update/remove action.
• The rejected form retains the learner's entered values and validation errors so it can be
  corrected and resubmitted.
• A persisted submission renders Submitted, its saved timestamp, and Update submission.
• Desktop and mobile project pages remain readable and have no horizontal overflow in both
  states.
• Focused Django, core/CI, quality, and browser evidence passes on a current-main candidate;
  the adoption manifest agrees with the changed copied files.

Dependencies

Uses the existing copied project behavior from #30. The later Course → Cohort work in #51/#56
must preserve this explicit persisted-state contract.

Existing evidence to reattach

The preserved candidate in .tmp/project-submission-status-current-20260827 was implemented
before this issue was formally groomed. It is not accepted automatically; it may be reused only
after the normal current-main engineer/tester/PM lifecycle is recorded against this issue.

Start with the project-page context and the copied project behavior from #30, then inspect the candidate evidence in .tmp/project-submission-status-current-20260827. Run the focused Django, core/CI, quality, and browser evidence on current main; done means rejected submissions remain Not submitted with values and errors, persisted submissions show their saved state, both layouts remain readable, and the adoption manifest matches changed copied files.