[Agent] PTZ: UI to save presets and set home position #658

Problem and evidence

The live-view PTZ controls (web/js/components/preact/PTZControls.jsx) can only use presets that already exist. The backend can create presets and set home, but the UI exposes neither:

• PUT /api/streams/{name}/ptz/preset with body {"name": "<preset name>"} saves the camera's current position as a new ONVIF preset and returns its token (src/web/api_handlers_ptz.c, handle_ptz_set_preset).
• POST /api/streams/{name}/ptz/set-home saves the current position as home (handle_ptz_set_home).
• Both require AUTHZ_PTZ_CONTROL and are audit-logged. Routes are in src/web/libuv_api_handlers.c:229-238.
• The UI already has: move, stop, home, GET /ptz/presets, and POST /ptz/goto-preset (fixed in 38e90865).

Expected behavior and scope

• In the PTZ panel, add "Save current position as preset…" (asks for a name, then refreshes the preset list on success) and "Set current position as home" (with confirmation).
• Hide or disable both when GET /ptz/capabilities says presets or home aren't supported (check which capability fields exist), or when the user gets a 403.
• Show success and failure with the existing toast pattern (live.ptzPresetFailed-style keys).
• New strings go in en.json under live.*.
• Optional, only if the backend already supports it: removing a preset. Don't add backend routes in this issue.

Acceptance criteria

• Saving a preset adds it to the dropdown without a page reload, and "Go to preset" then moves the camera there.
• Set-home followed by Home returns to the new position.
• Buttons are keyboard accessible with aria labels (jsx-a11y lint passes).

Validation

• cd web && npx vite build, npm run lint, npm test.
• Jest test with fetch mocked, checking method, URL and body for both actions and the preset-list refresh.
• Manual test on a real ONVIF PTZ camera if one is available. Mention in the PR if it was untested on hardware.

Constraints and rollback

• Frontend only.
• Rollback: revert.