fix(desktop): global box-sizing + per-pane error boundary + Conversation defensives

Two bugs reported after v2.2.0:

1. Input fields and textareas overflowed their container — typing in
   Settings / SendModal / NewContactModal would push the border past
   the card edge because the renderer's default box-sizing was
   content-box and `width: 100%` + padding pushed widths past parents.
   Added `*, *::before, *::after { box-sizing: border-box; }` to
   index.html. Removes the need for per-element `boxSizing: 'border-box'`
   (the existing sprinkles stay for clarity but are now redundant).

2. App went blank when opening a chat — any throw inside Conversation
   propagated up through Shell and wiped the whole window, with no way
   to navigate out. Added PaneBoundary, a React error boundary scoped
   to one Shell pane, keyed on `${section}-(list|detail)` so it resets
   when the user switches section. Now a crash shows an inline error
   card with message + stack + Retry, while NavBar + StatusBar stay
   usable.

   Also hardened Conversation against edge cases that were candidates
   for the original crash:
     * `name` always falls back to shortAddr(address) if all other
       branches produce an empty string.
     * first letter used for the avatar is computed once, guarded
       against empty input with a `?` fallback.
     * Header name + short-address line get whiteSpace/overflow/ellipsis
       so very long contacts no longer escape the 32px wide sub-column
       the way they did for the reporter.

Fonts normalised in the global CSS too — inputs/textareas/buttons now
inherit `font-family` instead of the browser default, which was
breaking the visual rhythm in the Settings cards.

desktop/src/shell/PaneBoundary.tsx

@@ -0,0 +1,62 @@
+// PaneBoundary — ErrorBoundary scoped to one Shell pane. A crash in
+// the Conversation component shouldn't black-out the whole window; it
+// should leave NavBar + List + StatusBar usable so the operator can
+// switch sections and report the bug. Resets when the keyed section
+// changes.
+
+import React from 'react';
+
+interface Props {
+  /** Used as React key at the callsite; also shown in the panic copy. */
+  sectionName: string;
+  children: React.ReactNode;
+}
+
+interface State {
+  error: Error | null;
+}
+
+export class PaneBoundary extends React.Component<Props, State> {
+  state: State = { error: null };
+
+  static getDerivedStateFromError(error: Error): State {
+    return { error };
+  }
+
+  componentDidCatch(error: Error, info: React.ErrorInfo): void {
+    console.error(`[PaneBoundary:${this.props.sectionName}]`, error, info);
+  }
+
+  render(): React.ReactNode {
+    if (!this.state.error) return this.props.children;
+    return (
+      <div style={{
+        padding: 20, height: '100%', overflow: 'auto',
+        background: '#000', color: '#fff', fontFamily: 'monospace',
+      }}>
+        <div style={{
+          color: '#ff6b6b', fontSize: 14, fontWeight: 700, marginBottom: 8,
+        }}>
+          {this.props.sectionName} crashed
+        </div>
+        <div style={{ color: '#fff', fontSize: 13, marginBottom: 8 }}>
+          {this.state.error.message}
+        </div>
+        <pre style={{
+          color: '#8b8b8b', fontSize: 11, lineHeight: 1.4,
+          whiteSpace: 'pre-wrap', wordBreak: 'break-word',
+        }}>
+          {this.state.error.stack}
+        </pre>
+        <button
+          onClick={() => this.setState({ error: null })}
+          style={{
+            marginTop: 10, padding: '6px 12px', borderRadius: 999,
+            border: '1px solid #1f1f1f', background: '#111',
+            color: '#fff', fontSize: 12, cursor: 'pointer',
+          }}
+        >Retry</button>
+      </div>
+    );
+  }
+}