feat: keep the chat tabs you left open, and keep them with you

Reopening the app closed every project tab: the copilot's tab bar lived in
RAM, so a reload dropped it and each conversation had to be found again from
its project board.

The set of open tabs is now a column on the session row, `chat_sessions.is_open`
(additive, `ensure_column`), restored by `GET /api/sessions/open` and written by
`PUT /api/sessions/{id}/open`. Not localStorage: that store is per-origin, so on
a shared laptop one member's tabs would greet the next, whereas the owner table
sits in their own encrypted file and follows them to another device. Which tab
is *selected* stays in sessionStorage — that one is genuinely per window, and a
shared value would have two windows fighting over it.

`is_open` defaults to 0 and `chat_sessions::create` never sets it: every `/new`
leaves its predecessor behind and every system-agent pass mints a row, so the
opposite default would restore a bar full of conversations nobody opened. Only
the copilot writes the column, at the moment it opens the tab. A reset moves the
flag rather than copying it — `POST /api/sessions` now returns the new id and
`new_session` carries it, and the old row is closed as the new one opens, or the
source would restore twice and a later close would clear the stale row.

Closing a tab clears the flag and nothing else: the conversation is kept and
comes back with its history when the project is reopened.

docs/projects.md

@@ -20,6 +20,8 @@ Opening a project shows its page, with two tabs (the current tab is part of the
 
 The header also has an **Open chat** button: it opens the project's conversation with the assistant. The assistant already knows the project folder and works directly inside it — creating documents, searching, summarizing. Each member has their **own private** conversation about the project; only the files are shared.
 
+The conversation opens as a **tab** in the chat panel, next to the General one. Those tabs stay open: they survive a page reload, and because they are saved to your account rather than to the browser, you find the same ones when you sign in from another device. Closing a tab only removes it from the bar — the conversation itself is kept, and reopening the project brings it back with its history. The General tab is always there and cannot be closed.
+
 ## The Files tab
 
 A file explorer rooted at the project folder: