Add back-photo button; consolidate reset/manage onto one hold-duration button

Back button (new GPIO0, POST /frame/back): the server now tracks a
bounded history of previously-current photos (photo_queue.py), pushed
to on every advance (auto or forced) and popped by back_forced() --
symmetric with advance, so pressing next afterwards returns to right
where you were. frame_client.c's force_advance bool becomes a 3-way
fetch_action_t (NORMAL/ADVANCE/BACK) threaded through the whole fetch
path.

Also folds the separate reset and manage buttons onto one pin
(combo_button.c, replacing reset_button.c/manage_button.c entirely),
disambiguated by hold duration: quick press shows the management menu
(unchanged), ~3s hold-then-release soft-resets (esp_restart(), config
kept -- new), ~15s hold factory-resets (today's old reset behavior,
extended from 10s for clearer tier separation). Driven by a production
board (Seeed XIAO ESP32-C6) exposing only 3 of the ESP32-C6's 8
deep-sleep-wakeup-capable GPIOs -- next/back keep their own dedicated
pins where instant response matters most, everything else shares the
third pin via timing instead of needing its own. Same three-pin layout
now works on both the dev board and the production board.

Fixed a fast-tap bug in combo_button_check() before shipping: it only
did a live gpio_get_level() read to decide whether the button was
pressed at all, so a press fast enough to already be released by the
time boot reached that check was missed entirely (treated as "never
pressed" rather than "quick press"). Added the same latched
esp_sleep_get_gpio_wakeup_status() check the other buttons already use
for exactly this reason.

server/app/main.py

@@ -130,6 +130,7 @@ def api_config_save(
         cfg.current_asset_set_at = 0.0
         cfg.queue = []
         cfg.queue_cursor = 0
+        cfg.history = []
     cfg.album_id = album_id
     cfg.order = order if order in ("sequential", "shuffle") else "sequential"
     cfg.refresh_interval_s = max(MIN_REFRESH_INTERVAL_S, min(MAX_REFRESH_INTERVAL_S, refresh_interval_s))
@@ -211,6 +212,26 @@ def frame_advance():
     return Response(content=_render_asset(client, cfg, cfg.current_asset_id), media_type="application/octet-stream")
 
 
+@app.post("/frame/back", dependencies=[Depends(require_access_token)])
+def frame_back():
+    """Returns to the previously-current photo (the mirror image of
+    /frame/advance -- see photo_queue.back_forced()), and resets the
+    interval clock from now. A no-op (still 200, current photo
+    unchanged) if there's no history to go back to -- same "always
+    returns something displayable" contract as /frame/advance, rather
+    than erroring. Used by the device's back-photo button."""
+    cfg = config.load()
+    _require_configured(cfg)
+
+    client = ImmichClient(cfg.immich_url, cfg.immich_api_key)
+    assets = _list_assets(client, cfg)
+
+    photo_queue.back_forced(cfg, assets)
+    config.save(cfg)
+
+    return Response(content=_render_asset(client, cfg, cfg.current_asset_id), media_type="application/octet-stream")
+
+
 LOCATION_LINE_MAX_LEN = 14
 
 US_STATE_ABBR = {