Fahrgestellnummer: expliziter "automatisch"-Schalter statt Nur-wenn-leer-Heuristik

- Neues Profilfeld fahrzeug.fin_automatisch (Standard: an).
- identitaet.py: bei "aus" läuft die Ableitung (samt Mehrfach-Sensor-Abgleich)
  gar nicht erst; bei "an" überschreibt der abgeleitete Wert auch einen
  vorher von Hand eingetragenen - der Schalter selbst entscheidet jetzt, statt
  aus einem leeren Feld zu raten.
- dienste.py: profil_schreiben löst die Ableitung jetzt auch aus, damit das
  Umschalten des Schalters selbst sofort wirkt.
- Panel: Schalter "automatisch" zwischen Label und Textfeld in der
  Fahrgestellnummer-Zeile, Feld wird bei "an" deaktiviert.

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

AGENTS.md

@@ -4444,6 +4444,44 @@ needed either - the "Fahrgestellnummer (FIN)" field in "Fahrzeug einrichten" alr
 profile field, so it now simply arrives pre-filled instead of blank, and stays fully editable/correctable by
 hand same as before.
 
+**Same-day follow-up (2026.8.28.16): an explicit "automatisch" switch, per owner request** ("make a Slider
+next to 'Fahrgestellnummer (FIN)' ... automatisch turns on/off the auto-grab VIN from Sensor-Database ...
+and also deactivates the sanity-check if VIN matches between sensors"). The original design's "only fill an
+empty field, never touch a set one" heuristic is gone - replaced by an explicit, user-visible mode:
+
+- New profile field `fahrzeug.fin_automatisch` (bool, default `true` - added to `vorlage/fahrzeugprofil.json`
+  and defaulted in `profilZuCar()` via `!== false` for profiles from before this field existed).
+- `identitaet.fin_ableiten_und_uebernehmen()` rewritten: returns immediately if `fin_automatisch` is `false`
+  - **before** even calling `fin_kandidaten_ermitteln()`, so the multi-sensor comparison/mismatch-warning
+  never runs either, exactly as asked. When `true`, the derived value is now treated as authoritative and
+  **overwrites** whatever is currently in `fin` (including a previously hand-typed value) whenever a single
+  clear candidate is found - this is the whole point of an explicit toggle: no more guessing from emptiness,
+  the human's chosen mode decides. A `fin == kandidat` check avoids a pointless rewrite/republish when
+  nothing actually changed.
+- Panel (`vEinst()`): a `<label class="switch">` reading "automatisch" placed between the "Fahrgestellnummer
+  (FIN)" label and the text input, in the same `.feld` row (owner's exact layout request) - the input gets
+  `disabled` whenever automatic is on (`.feld input:disabled{opacity:.5;cursor:not-allowed}`, new rule) and
+  a title tooltip explaining why, and re-enables the moment automatic is switched off. Wired via the same
+  `data-*`/central-`change`-listener idiom as every other switch in this file (mirrors `data-steuerlast`).
+  `CAR.finAutomatisch`/`p.fahrzeug.fin_automatisch` added to both directions of the profile↔CAR mapping.
+- `dienste.py`'s `profil_schreiben` handler (not just `entitaeten_schreiben`) now also calls
+  `identitaet.fin_ableiten_und_uebernehmen()` after writing - this is the actual mechanism that makes
+  flipping the switch itself take effect immediately: toggling it goes through the ordinary profile-save
+  path (same as every other "Fahrzeug einrichten" field), so the just-written `fin_automatisch` flag is
+  what the very next derivation attempt reads.
+
+**Verified live end-to-end, all four transitions, not just read from source**: (1) automatic on, empty
+underlying state → real VIN appears, log line fires; (2) toggled off → input's `disabled`/`opacity`
+computed style confirmed `false`/`1` (genuinely editable, not just visually similar - the app's normal
+`.feld input` background already looks grey by design, so a screenshot alone wasn't proof); (3) typed a
+fake value while off, saved → persisted to disk exactly as typed, confirmed via direct file read, log
+stayed silent (sanity-check correctly skipped); (4) toggled back on → immediately overwrote the fake value
+back to the real derived VIN, both the on-disk file and a fresh log line (`Fahrgestellnummer automatisch
+... übernommen: WUAZZZF48PA902804`) confirmed within seconds, no manual save/reload needed. Layout also
+confirmed clean at both a real 1400px desktop width and the default phone-width layout - no overflow at
+either. `node --check` clean, manifest bumped to `2026.8.28.16`, `audi_ha_test` restarted and confirmed
+clean via log.
+
 Verified live in `audi_ha_test` at every step (`.6` then `.7`), each time reading the actual rendered Setup
 popup fresh after a real restart (not a stale tab - confirmed via `performance.getEntriesByType('resource')`
 showing the new `?v=` served with a nonzero transfer size each time). Final state (`.7`), all 20 single-value