fix(devices): pairing input moves into the Add Device modal

The pairing field sat at the bottom of the Devices page, below the
device grid — unreachable on any account with more than a screenful of
devices (the user's dev account has ~480 accumulated test devices;
scrolling 224k px to reach the field is a non-starter). The Add Device
button at the top of the page is where pairing intent already lives,
so the code input and Find device button now sit inside the Connect a
KOReader Device modal as instruction step 6, with the confirm card
swapping into the same modal (container keeps the pairing-lookup ids;
the htmx error-toast listener scopes to #add-device-modal now).

Verified live: modal renders the full instruction list + input, a
wrong code swaps the templ error partial inside the modal, and claim
still HX-Refreshes on success (modal state resets with the reload,
paired device appears in the grid).

ui-controls glossary where-fields and the COVERAGE row updated with
the control's new home.

docs/developer/ui-controls.md

@@ -183,11 +183,11 @@ where: All Books / Podcasts page → toolbar
 does: Opens the filter drawer for building and applying rules.
 
 label: Find device
-where: Devices page → Pair a Device
+where: Devices page → Add New Device modal
 does: Looks up the pairing code shown on the registering e-reader and renders the confirm card.
 
 label: Pair device
-where: Devices page → Pair a Device → confirm card
+where: Devices page → Add New Device modal → confirm card
 does: Binds the device behind the pairing code to the signed-in account.
 
 label: Download All