Fix code-review findings across domain, persistence, server and frontend

Full-codebase review turned up five real correctness/security bugs and
a dozen smaller inconsistencies; all are fixed here with matching test
coverage:

- BMK uniqueness silently allowed German-umlaut duplicates ("Ä1" vs
  "ä1") because the DB's normalized index only folds ASCII case. Added
  a shared Unicode-aware pre-check used by every circuit/component
  insert and rename path (one of which had no pre-check at all).
- CircuitDeviceRow.simultaneityFactor had no upper bound at the row
  level (command model and snapshot/restore schema), unlike every
  sibling entity, letting a bad value silently corrupt power totals.
- Grid cell editing silently misread German thousands-separator input
  ("1.500" parsed as 1.5); "." is now rejected outright with a clear
  message instead of guessing.
- The editor's shared command runner (runCommand/applyHistory) had no
  re-entrancy guard, so a double click/drop could fire the same
  command twice and race a BMK collision or revision conflict. Added a
  synchronous ref guard plus isSaving on the buttons that lacked it.
- GET .../next-identifier leaked circuit-numbering state for sections
  in other projects (no ownership check, 400 instead of 404). Moved
  under /projects/:projectId and scoped it.

Also: added the missing circuits.section_id / circuit_device_rows.
circuit_id indexes (migration 0006), gave FormModal a focus trap /
Escape-to-close / focus restore and rebuilt ProjectSettingsModal on
top of it instead of duplicated markup, removed dead code (3 orphaned
domain model files, an unused persistence helper, a wrapper only used
by its own test), pointed the project page at GET /projects/:id
instead of listing+filtering client-side, closed the gap between the
documented 18 MB CSV limit and the ~17.17 MiB actually enforced, added
missing upper bounds on several free-text fields, filled in nine
missing German labels in the revision timeline, replaced a
key-order-fragile JSON.stringify equality check with a real field
comparison, made an implicit sort-order assumption in three
renumbering helpers explicit, cleared the sidebar's target selection
when it no longer resolves after a tree reload, and fixed
updateGlobalDevice to check-then-write instead of write-then-check.

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

src/frontend/components/form-modal.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import React, { type FormEvent, type ReactNode } from "react";
+import React, { type FormEvent, type ReactNode, useEffect, useRef } from "react";
 
 interface FormModalProps {
   children: ReactNode;
@@ -14,6 +14,9 @@ interface FormModalProps {
   title: string;
 }
 
+const FOCUSABLE_SELECTOR =
+  'a[href], button:not([disabled]), textarea:not([disabled]), input:not([disabled]), select:not([disabled]), [tabindex]:not([tabindex="-1"])';
+
 export function FormModal({
   children,
   description,
@@ -25,11 +28,56 @@ export function FormModal({
   submitLabel,
   title,
 }: FormModalProps) {
+  const dialogRef = useRef<HTMLDivElement>(null);
+
+  // Focuses the dialog on open and returns focus to the element that
+  // triggered it on close, so keyboard users never lose their place in the
+  // grid behind the backdrop.
+  useEffect(() => {
+    const previouslyFocused = document.activeElement as HTMLElement | null;
+    const firstFocusable =
+      dialogRef.current?.querySelector<HTMLElement>(FOCUSABLE_SELECTOR);
+    firstFocusable?.focus();
+    return () => {
+      previouslyFocused?.focus();
+    };
+  }, []);
+
+  function handleKeyDown(event: React.KeyboardEvent<HTMLDivElement>) {
+    if (event.key === "Escape") {
+      if (!isSaving) {
+        event.stopPropagation();
+        onClose();
+      }
+      return;
+    }
+    if (event.key !== "Tab" || !dialogRef.current) {
+      return;
+    }
+    const focusable = Array.from(
+      dialogRef.current.querySelectorAll<HTMLElement>(FOCUSABLE_SELECTOR)
+    );
+    if (focusable.length === 0) {
+      return;
+    }
+    const first = focusable[0];
+    const last = focusable[focusable.length - 1];
+    if (event.shiftKey && document.activeElement === first) {
+      event.preventDefault();
+      last.focus();
+    } else if (!event.shiftKey && document.activeElement === last) {
+      event.preventDefault();
+      first.focus();
+    }
+  }
+
   return (
     <>
       <div
         aria-modal="true"
         className="modal fade show d-block"
+        onKeyDown={handleKeyDown}
+        ref={dialogRef}
         role="dialog"
         tabIndex={-1}
       >