feat(marking): print stylesheet + .json session save/load (nightly-build threshold)

[hyperpolymath]

Stacked PR (9 of 9 follow-ups). Base = `feature/composer-rubric-editor` (PR #59). Merge order: #51 → #52 → #53 → #54 → #55 → #56 → #57 → #58 → #59 → this. PR #60 (finch lock) can land independently and in any order — it only affects deps.

Summary

Pushes the composer to the threshold of "first nightly build": output is no longer just two textareas to copy-paste — it can be printed cleanly and saved/restored as a portable .json artefact.

Print

• New `@media print` block in `assets/css/app.css` with composer-specific classes (`composer-print-hide` / `composer-print-page`) so the interactive form, header and aside are hidden in print mode, and a dedicated paper-friendly layout takes their place.
• Print page shows:
  • Student name + band + rubric summary + generation timestamp
  • The manual-override warning (when present)
  • Both prose boxes as paragraph-flowed text
  • Per-component breakdown as a table
  • A borderline-review list if any flags fired
• New Print button next to "Clear / new student" — uses a JS hook + `window.print()`.

Session save/load

• Save session (.json) bundles current `form_state` + outputs (`did_well`, `improve`, `band`) and triggers a browser download. Filename: `exam-feedback[-Student]-.json`.
• Open session (.json) opens a hidden file input; selecting a file pushes `restore_session` to the server, which normalises `form_state` through the existing `normalize_form_params` path (so a tampered file is re-validated) and restores the cached outputs.
• Schema:
  ```json
  {
  "schema_version": 1,
  "saved_at": "2026-06-10T07:42:00Z",
  "form_state": { /* full normalised form params */ },
  "outputs": {
  "did_well": "...",
  "improve": "...",
  "band": {"key": "good", "label": "good / Pass 2", "mark": 72}
  }
  }
  ```
• Atom safety: `map_to_band/1` uses `String.to_existing_atom` + `rescue` so a malformed band key cannot poison the BEAM atom table.

What this closes

With PRs #51–#59 + #60 (deps) + this:

• Composer renders, persists across refresh (localStorage), saves/loads as portable file, prints cleanly, exposes rubric editor + reload.
• The "save / print / load" foundations called out in the previous turn are all wired.

What this does not close

• Browser walkthrough still pending — verified at module level only (68 tests). The `mix phx.server` workflow needs the `finch` bump (chore(deps): bump finch 0.20 -> 0.22 to satisfy req 0.5.18 #60) + a working local OTP install with `syntax_tools` (separate, environment-dependent).
• TMA-pipeline integration (FHI ingestion / .docx export / CubDB) — separate Phase 1 work, weeks of effort.

Test plan

• LiveView parses cleanly
• 68 composer module tests still pass
• Browser: Generate feedback → Save session → reopen tab → Open session → form + outputs restored
• Browser: Print → preview shows the dedicated print page (paper-style); interactive chrome hidden
• Atom-safety: open a session JSON with a bogus `band.key` → load completes, band shown as nil rather than crashing
• Save with no student name → filename falls back to `exam-feedback-.json`

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