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.

web/components/projects/project-board.js

@@ -139,9 +139,9 @@ export class ProjectBoardSection extends LightElement {
     try {
       const res = await fetch(`/api/projects/${this._projectId}/session`, { method: 'POST' });
       if (!res.ok) throw new Error(await res.text());
-      const { source } = await res.json();
+      const { source, session_id } = await res.json();
       window.dispatchEvent(new CustomEvent('project-chat-open', {
-        detail: { source, label: this._project?.name ?? `Project ${this._projectId}` },
+        detail: { source, session_id, label: this._project?.name ?? `Project ${this._projectId}` },
       }));
     } catch (e) {
       this._error = e.message;