Sova v0.5.0

Chat powers, a Folders sidebar and steadier chat starts. 53 commits since v0.4.0. The highlights:

• Switches replace overseer levels. A project overseer's powers are On/Off switches in two
  columns, what it may do on its own and what the client's Project Assistant may set off, instead
  of levels L0–L3. Your old level carries over (what it allowed is On, the rest Off). While a brake
  holds, every switch acts as Off and draws held; what you set is kept.
• Every chat can read the project's spec. Chats look up the product documentation on their own,
  and each conversation that gathers requirements writes its own spec draft, locked in only by an
  explicit yes.
• The owner page gets the Project Assistant. Versions to try behind the owner link, models per
  kind of work, outage alerts, and an owner link that turns off after 7 days unused.
• Folders replaces Live & web. The sidebar's top region is called Folders, sits under Projects
  and folds like it; a session a project already lists isn't repeated there.
• Chats start more reliably. A slow start names the step it is stuck in, a late hello no longer
  gives up early, and a reader who scrolls up while a chat streams stays where they are.
• One update banner. Every tab learns that Sova changed through the same banner, mesh on or off;
  the bottom-right "Sova was updated" note is gone.
• Smaller fixes. Less idle CPU, no Review Changes on a branch without commits of its own, and
  the changes viewer refuses an empty comparison instead of showing a different change.

Going back to v0.4.0 (SOVA_REF=v0.4.0) hasn't been tested on this release: each project overseer
runs at a level Sova saved beside its switches, never looser than they were, so it may be allowed
less than here; the switches stay stored and return when you upgrade again.