lint: drive baseline to zero, drop only-new-issues gate

Cleanup pass over the repo so CI can enforce lint going forward
without the only-new-issues escape hatch:

* gofumpt -w across the tree (31 hits, all formatting)
* misspell --fix (25 hits, US-locale spelling) — but reverted on
  api.JobCancelled = "cancelled" since that literal is the wire +
  DB CHECK constraint value, plus matched the case in store/fleet.go
  back to "cancelled" and added //nolint:misspell on both for the
  next time someone reaches for the auto-fix
* Wrap every `defer rows.Close()` / `defer stmt.Close()` /
  `defer res.Body.Close()` in `defer func() { _ = .Close() }()`
  to satisfy errcheck without losing the close itself
* websocket.Dial callers (1 prod, 4 tests) now capture + close the
  upgrade response Body — coder/websocket can return res with a nil
  Body on success, so the test deferred-closes guard against that
* Annotate the two genuine-by-design nilerr cases with //nolint
  comments explaining why nil-on-error is the contract (cookie
  missing = no session; ctx cancelled mid-backoff = clean shutdown)
* Add brief godoc on the 10 exported const groups + types that
  revive flagged (api.HostOS/HostArch/JobKind/JobStatus/LogStream/
  ErrorCode, restic.EventKind, store.Role, web.FS)
* Drop the unused (*Server).userByID method
* Inline the unparam baseView(active) — every UI page is under
  the dashboard primary nav today

Result: `golangci-lint run ./...` reports 0 issues. CI lint job
no longer needs only-new-issues: true; X-06 follow-up entry in
tasks.md removed.

internal/server/http/ui_schedules.go

@@ -78,7 +78,7 @@ func (s *Server) handleUISchedulesList(w stdhttp.ResponseWriter, r *stdhttp.Requ
 	chrome.ScheduleCount = len(scheds)
 	chrome.SourceGroupCount = len(groups)
 
-	view := s.baseView(u, "dashboard")
+	view := s.baseView(u)
 	view.Title = host.Name + " schedules · restic-manager"
 	view.Page = hostSchedulesPage{
 		hostChromeData: chrome,
@@ -106,7 +106,7 @@ func (s *Server) handleUIScheduleNewGet(w stdhttp.ResponseWriter, r *stdhttp.Req
 		stdhttp.Error(w, "internal", stdhttp.StatusInternalServerError)
 		return
 	}
-	view := s.baseView(u, "dashboard")
+	view := s.baseView(u)
 	view.Title = "New schedule · " + host.Name + " · restic-manager"
 	view.Page = scheduleEditPage{
 		hostChromeData:   s.loadHostChrome(r, *host, "schedules", "new schedule"),
@@ -152,7 +152,7 @@ func (s *Server) handleUIScheduleEditGet(w stdhttp.ResponseWriter, r *stdhttp.Re
 	for _, gid := range sc.SourceGroupIDs {
 		selected[gid] = true
 	}
-	view := s.baseView(u, "dashboard")
+	view := s.baseView(u)
 	view.Title = "Edit schedule · " + host.Name + " · restic-manager"
 	view.Page = scheduleEditPage{
 		hostChromeData: s.loadHostChrome(r, *host, "schedules", "edit schedule"),
@@ -381,7 +381,7 @@ func (s *Server) renderScheduleFormError(w stdhttp.ResponseWriter, r *stdhttp.Re
 		saveAction = "/hosts/" + host.ID + "/schedules/" + sid + "/edit"
 		crumb = "edit schedule"
 	}
-	view := s.baseView(u, "dashboard")
+	view := s.baseView(u)
 	view.Title = "Schedule · " + host.Name + " · restic-manager"
 	view.Page = scheduleEditPage{
 		hostChromeData:   s.loadHostChrome(r, *host, "schedules", crumb),