feat: add restore/unarchive for archived sessions

Archived sessions had no way back to the active list: the only available
action was "Delete permanently". Add restore per session (sidebar context
menu, Archive page row) and in bulk (sidebar selection bar).

The OpenCode server cannot clear time.archived over HTTP — session.update
only applies the field for a finite number, so an omitted key is a no-op
and null is silently ignored (verified against opencode 1.18.12). Restore
therefore writes time.archived = 0: every client-side reader classifies
archive state by truthiness, so 0 reads as active in the UI, the event
reducer, and the OpenCode app/TUI.

The server's time_archived IS NULL list filter still excludes such rows,
so the global session cache no longer issues an archived:false request
for its active list. Full and per-directory loads now fetch once with
the inclusive flag and split client-side via splitGlobalSessionsByArchived,
which also halves per-directory refresh requests. Directory bootstrap
keeps the server filter because live child stores must not hold archived
sessions; a restored session re-enters its live store through the
authoritative session.updated event.

unarchiveSession/unarchiveSessions follow the archiveSession contract:
wait for server confirmation before reconciling stores, runtime-guard
every reconciliation, preserve partial batch results, and fail loudly
when the server keeps the session archived instead of toasting a
successful no-op.

Closes #2346

packages/ui/src/stores/DOCUMENTATION.md

@@ -59,8 +59,8 @@ User-visible session ordering is also not owned by the global cache array order.
 
 Global refresh rules:
 
-- The OpenCode `archived` list flag means "also include archived sessions": the server only drops its `time_archived IS NULL` condition. `listGlobalSessionPages` therefore narrows archived requests to records carrying `time.archived`, at the data boundary, so the archived cache never holds active sessions and no consumer has to re-derive that. Pagination progress stays measured on the raw response, so a page that is full upstream but filtered out here is not mistaken for the last page.
-- Per-directory refresh is bounded to two requests across callers and prioritizes the current directory.
+- The OpenCode `archived` list flag means "also include archived sessions": the server only drops its `time_archived IS NULL` condition. The global cache therefore loads with one inclusive request (`archived: true`) and splits active/archived client-side via `splitGlobalSessionsByArchived` — an `archived: false` request cannot be truthful because the server filter excludes restored sessions (`time.archived` falsy-but-present, see "Restore (unarchive) contract" in `sync/DOCUMENTATION.md`). For callers that still want only archived records, `listGlobalSessionPages` narrows inclusive responses at the data boundary (default `narrowToArchived`), so the archived cache never holds active sessions and no consumer has to re-derive that. Pagination progress stays measured on the raw response, so a page that is full upstream but filtered out here is not mistaken for the last page.
+- Per-directory refresh issues one inclusive request per directory (previously two), bounded to two requests across callers and prioritizing the current directory.
 - Each directory is an independent completeness scope. A failed directory preserves its previous sessions while successful directories reconcile normally.
 - Fetch failure must remain distinguishable from a successful empty list; failed scopes cannot destructively clear cached sessions.
 - Runtime switch increments the load generation and clears the previous runtime's snapshot so stale in-flight work cannot commit.