feat(projects): drag the sidebar into the order you want

Projects have carried sort_order since the first migration and nothing
ever set it, so the sidebar was ordered by creation date for good.

POST /projects/reorder takes the whole list rather than one move. A
person has a handful of projects, so sending the order you want is
simpler to reason about than midpoints between neighbours — and it cannot
run out of precision, which is the failure the task ordering has to
respace a section to recover from. It also cannot land half-applied the
way a sequence of individual moves can.

Projects you do not own are skipped rather than refused. sort_order is a
column on the project, not a preference per viewer, so a shared project
sits where its owner put it and a member rearranging their own sidebar
does not rearrange somebody else's. The sidebar only makes your own
projects draggable, so that rule never shows up as a row that springs
back.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/httpapi/server.go

@@ -259,6 +259,8 @@ func New(cfg *config.Config, db *store.DB, log *slog.Logger, web fs.FS) *Server
 			r.Route("/projects", func(r chi.Router) {
 				r.Get("/", s.handleListProjects)
 				r.Post("/", s.handleCreateProject)
+				// Above /{projectID}, or chi reads "reorder" as an id.
+				r.Post("/reorder", s.handleReorderProjects)
 
 				r.Route("/{projectID}", func(r chi.Router) {
 					// Read access is the floor for the whole subtree; the writes