P2R-02 slice 2: Sources tab — list, new/edit form, delete, Run-now

Sources tab now lists every source group on the host with per-row
counts (used-by-N-schedules, snapshot count by tag), the v4
conflict tag (keep-* dimension that has no compatible cadence),
and Run-now / Edit / Delete actions. Run-now reuses the existing
HTMX-aware /hosts/{id}/source-groups/{gid}/run handler.

New /hosts/{id}/sources/new and /sources/{gid}/edit form: name +
includes/excludes textareas + the 3×2 keep-* retention grid +
retry-on-offline knobs. Server-side validation re-renders with the
operator's input intact; the inline conflict banner shows above the
retention grid when ConflictDimension is set.

Delete blocks (UI + server) when the group is referenced by any
schedule. Every successful mutation calls pushScheduleSetAsync so
an online agent re-arms within seconds.

Adds .src-row and .keep-cell to input.css for the row + retention
grid layout.

internal/server/http/server.go

@@ -185,8 +185,13 @@ func (s *Server) routes(r chi.Router) {
 		r.Get("/hosts/pending/{token}/awaiting", s.handleUIPendingAwaiting)
 		// Host detail (Snapshots tab is the default).
 		r.Get("/hosts/{id}", s.handleUIHostDetail)
-		// Sources tab (slice 2 fills in CRUD).
+		// Sources tab + source-group CRUD forms.
 		r.Get("/hosts/{id}/sources", s.handleUIHostSources)
+		r.Get("/hosts/{id}/sources/new", s.handleUISourceGroupNewGet)
+		r.Post("/hosts/{id}/sources/new", s.handleUISourceGroupSave)
+		r.Get("/hosts/{id}/sources/{gid}/edit", s.handleUISourceGroupEditGet)
+		r.Post("/hosts/{id}/sources/{gid}/edit", s.handleUISourceGroupSave)
+		r.Post("/hosts/{id}/sources/{gid}/delete", s.handleUISourceGroupDelete)
 		// Repo tab (slice 4 fills in body).
 		r.Get("/hosts/{id}/repo", s.handleUIHostRepo)
 		// Schedules tab + create/edit/delete forms.