Add manual scheduled task entry

Lets users log tasks the Linux agent can't discover on its own (e.g.
Docker-based backup jobs) directly in the UI. Tasks now carry an
origin ('agent' | 'manual') so the agent's report-sync logic only
ever creates/updates/stale-marks agent-sourced rows, leaving manual
entries untouched; the API rejects edits/deletes of agent-sourced
tasks to keep that boundary enforced server-side too.

web/tsconfig.tsbuildinfo

@@ -1 +1 @@
-{"root":["./src/app.tsx","./src/main.tsx","./src/vite-env.d.ts","./src/api/client.ts","./src/components/filterbar.tsx","./src/components/servergroup.tsx","./src/components/tasktable.tsx","./src/pages/dashboard.tsx","./src/pages/login.tsx","./src/pages/serversadmin.tsx"],"version":"5.9.3"}
+{"root":["./src/app.tsx","./src/main.tsx","./src/vite-env.d.ts","./src/api/client.ts","./src/components/filterbar.tsx","./src/components/servergroup.tsx","./src/components/taskform.tsx","./src/components/tasktable.tsx","./src/pages/dashboard.tsx","./src/pages/login.tsx","./src/pages/serversadmin.tsx"],"version":"5.9.3"}