Project save improvements

[rozyczko]

We need a way to notify users about the project state being dirtied and in need of save.
Additionally, saving prtoject should be improved:

• floppy disc icon should have its color changed when the project is dirty
• the icon should be grayed for clean state
• user should get some information about the project having been saved (status bar?)

[rozyczko]

Project save feedback — analysis and design proposal

Context: Saving the project (floppy disk icon, top-left of the main window) succeeds but gives
the user no confirmation. Question: is anything wired but not shown, or is nothing included at all?

Currently: nothing is wired. Failures are silent

The whole chain from the floppy button down is fire-and-forget:

1. Gui/ApplicationWindow.qml:37 → projectSave() →
   Gui/Globals/BackendWrapper.qml:79 → the save() slot in
   Backends/Py/project.py (Project.save) → ProjectLogic.save() → the lib's
   save_as_json(overwrite=True). No signal is emitted anywhere, nothing returns a result, and no
   QML element listens.

2. It is actually worse than "no confirmation": the lib's save_as_json
   (reflectometry-lib, project.py) wraps the write in
   except Exception as exception: print(exception) — a failed save (locked file,
   permissions, and notably the documented ValueError that as_dict() raises when a constraint
   depends on a parameter not reachable from the models) prints to a console the GUI user never
   sees. Today "saved silently" and "failed silently" look identical.

3. There is no dirty/unsaved-changes tracking anywhere (lib or app), so the app also cannot tell
   whether saving was even needed, and the reset/close paths can discard unsaved work without
   warning.

One reusable asset exists:
Gui/Pages/Summary/Sidebar/Basic/Groups/SaveConfirmationDialog.qml — the modal success/failure
dialog the Summary export already uses. EasyApp has no toast/snackbar component.

Proposed design

Phase 1 — honest feedback (small, recommended)

• Lib: stop swallowing exceptions in save_as_json (let them propagate, or return success).
  Without this, any confirmation the GUI shows would be a guess.
• Backend: Project.save() catches, then emits a new projectSaved(success, path, message)
  signal, plus a lastSaved timestamp property (notify-backed).
• UI, asymmetric by severity:
  • Failure → modal dialog (reuse the SaveConfirmationDialog pattern with the error message).
    A failed save must not be missable.
  • Success → no modal. A dialog on every routine save is friction; instead:
    1. flash the floppy icon to a check mark for ~2 s (in-place, exactly where the user just
       clicked), and
    2. a status-bar item "Saved · 14:32" showing the last save time persistently. The status bar
       already shows project/models/experiments, so it fits naturally and stays inspectable after
       the flash is gone.

Phase 2 — dirty tracking (bigger, best long-term)

A hasUnsavedChanges flag set from the existing change funnels in py_backend
(externalSampleChanged, parametersChanged, constraints/experiment signals all pass through one
place), cleared on save/load/create. That buys:

• an asterisk on the window title or a dot badge on the save button,
• save button disabled when clean (making "it saved" implicit),
• an "unsaved changes" prompt on Reset and on window close: the reset dialog currently says
  "cannot be undone" without knowing whether anything would be lost.