Feature: make auto-save optional (manual save + Ctrl+S) with an unsaved-changes indicator

[tomasmorello]

Problem

Currently the editor saves the file on every change (debounced ~500ms). While
convenient, this means a half-finished or accidental edit is written to disk
immediately — there is no way to "try something" without persisting it nor undo. This has
led to me accidentally breaking config files.

Proposal

Add an opt-out for auto-save, keeping the current behaviour as the default:

• An Auto-save toggle in the editor toolbar (persisted in localStorage).
  Default: on, so nothing changes for existing users.
• When auto-save is off:
  • Save explicitly via a save button (disk icon) or Ctrl/Cmd+S.
  • Unsaved edits are kept as an in-memory draft, so switching tabs does not
    discard them.
  • Files with unsaved changes are flagged in the file tree (a small amber dot,
    kept visually distinct from the existing docker status indicator).
• Ctrl/Cmd+S also works with auto-save on (flushes the debounce immediately)
  and prevents the browser's default save dialog.

Notes / open questions

• Undo history is preserved across tab switches by giving each file its own
  Monaco model (keepCurrentModel). It is intentionally not persisted across
  a full page reload (Monaco has no public API to serialize the undo stack).
• Should manual mode be the default instead of auto-save? I kept auto-save as the
  default to avoid changing current behaviour, but happy to flip it.

I already have an implementation ready and can open a PR if this sounds good.
Tagging @RA341 for thoughts on the approach / defaults before I submit.