Redesign phase C: claim flow, limited manage page, device protocol

The frame-claiming pipeline, end to end. Firmware: every request now
carries ?id=<12-hex STA MAC> via build_url (mirrored in build_ota_url),
and the captive portal's success page became a redirect that hands the
user's browser to <server>/claim?device_id=... after ~7s -- enough time
for the phone to drop the provisioning AP while the device reboots.
The server pushes a per-frame device token through /frame/config during
a one-time handshake; the firmware persists it to NVS (a dedicated
single-key write that deliberately doesn't reset the connected-once
flag or WiFi cache) and prefers it over the provisioned shared token
from the next request on. Config response buffer grows 256->512. Both
board variants compile clean; new firmware also works against an old
server (which ignores ?id=) and old firmware against this server (the
phase A legacy mapping), so either deploy order survives.

Server: /claim lands the captive-portal redirect -- claim-gated signup
(a valid unclaimed/unregistered device id IS the enrollment invitation),
pending claims for the user-beats-the-frame race (auto-attached at
self-registration, 24h expiry), and a waiting page that refreshes until
the frame checks in. Unclaimed/unconfigured frames get a rendered
instruction placeholder with a QR from /frame/image (200, never an
error loop) -- new qrcode dep, placeholder shares the exact
quantize/pack path photos use.

The on-frame manage QR now resolves to a limited no-login page: scans
of / carrying device credentials (new ?id&token or the legacy shared
token) 303 to /m/<manage_token>, which allows exactly view queue,
show-next, advance, back, and scoped thumbnails -- no settings, no
removal, no other frames. Full control means logging in.

One real protocol hole found by simulating full wake cycles: after
self-registration the device could never authenticate again (the wake
cycle fetches the image BEFORE /frame/config delivers its token).
require_device now treats the id itself as the credential until the
first authenticated request flips device_token_ack -- the same trust
level as open registration, closing permanently once the handshake
completes.

firmware/main/root.html

@@ -98,10 +98,14 @@
             </div>
 
             <div class="input-group">
-                <label for="access_token">Access Token (optional)</label>
-                <input type="text" id="access_token" name="access_token" placeholder="only if the server's MANAGEMENT_TOKEN is set" maxlength="64">
+                <label for="access_token">Access Token (optional &mdash; only for older servers)</label>
+                <input type="text" id="access_token" name="access_token" placeholder="usually blank; current servers issue one automatically" maxlength="64">
             </div>
 
+            <p style="font-size: 13px; color: #555;">After saving, this page will
+                take you to the server to claim your frame &mdash; reconnect to
+                your normal WiFi if it doesn't happen automatically.</p>
+
             <button type="submit">Submit</button>
 
         </form>