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/sections/messages/Conversation.tsx

@@ -98,12 +98,13 @@ export function Conversation({ address }: { address: string }): React.ReactEleme
     }
   };
 
-  const name = contact?.username ? `@${contact.username}`
+  const name = (contact?.username ? `@${contact.username}`
     : contact?.alias
       ? contact.alias
       : isSelf
         ? 'Saved Messages'
-        : shortAddr(address, 8);
+        : shortAddr(address || '', 8)) || shortAddr(address || '', 8);
+  const firstLetter = (name || '?').replace(/^@/, '').charAt(0).toUpperCase() || '?';
 
   return (
     <div style={{ height: '100%', display: 'flex', flexDirection: 'column' }}>
@@ -118,12 +119,18 @@ export function Conversation({ address }: { address: string }): React.ReactEleme
           color: '#fff', fontWeight: 700, fontSize: 14,
           display: 'flex', alignItems: 'center', justifyContent: 'center',
         }}>
-          {isSelf ? '★' : name.replace(/^@/, '').charAt(0).toUpperCase()}
+          {isSelf ? '★' : firstLetter}
         </div>
         <div style={{ flex: 1, minWidth: 0 }}>
-          <div style={{ color: '#fff', fontSize: 14, fontWeight: 700 }}>{name}</div>
-          <div style={{ color: '#6a6a6a', fontSize: 11, fontFamily: 'monospace' }}>
-            {shortAddr(address, 6)}
+          <div style={{
+            color: '#fff', fontSize: 14, fontWeight: 700,
+            whiteSpace: 'nowrap', overflow: 'hidden', textOverflow: 'ellipsis',
+          }}>{name}</div>
+          <div style={{
+            color: '#6a6a6a', fontSize: 11, fontFamily: 'monospace',
+            whiteSpace: 'nowrap', overflow: 'hidden', textOverflow: 'ellipsis',
+          }}>
+            {shortAddr(address || '', 6)}
           </div>
         </div>
       </div>