feat(email): faster inbox load, Ctrl+Tab tab cycling, square preset buttons

- Inbox load: add an mtime-keyed in-memory cache to listInboxPage so it no
  longer re-reads and JSON.parses every cached thread (bodies included) on
  every call — only files whose mtime changed are re-parsed. Speeds up the
  Important→Everything-else handoff, live reloads during sync, and re-opening
  the inbox tab.
- Add Ctrl+Tab / Ctrl+Shift+Tab to cycle to the next/previous tab (wraps
  around), alongside the existing Cmd+Shift+] / [ shortcuts.
- Improve bar: make the tone preset buttons square (rounded-md), left-aligned,
  with a bit of vertical gap so wrapped rows aren't cramped.

apps/x/apps/renderer/src/App.tsx

@@ -4727,6 +4727,25 @@ function App() {
         }
         return
       }
+
+      // Ctrl+Tab — next tab, Ctrl+Shift+Tab — previous tab (browser-style).
+      // Bound to Ctrl specifically (Cmd+Tab is the OS app switcher on macOS).
+      if (e.ctrlKey && e.key === 'Tab') {
+        e.preventDefault()
+        const direction = e.shiftKey ? -1 : 1
+        if (inFileView) {
+          const currentIdx = fileTabs.findIndex(t => t.id === targetFileTabId)
+          if (currentIdx === -1) return
+          const nextIdx = (currentIdx + direction + fileTabs.length) % fileTabs.length
+          switchFileTab(fileTabs[nextIdx].id)
+        } else {
+          const currentIdx = chatTabs.findIndex(t => t.id === activeChatTabId)
+          if (currentIdx === -1) return
+          const nextIdx = (currentIdx + direction + chatTabs.length) % chatTabs.length
+          switchChatTab(chatTabs[nextIdx].id)
+        }
+        return
+      }
     }
     document.addEventListener('keydown', handleTabKeyDown)
     return () => document.removeEventListener('keydown', handleTabKeyDown)

apps/x/apps/renderer/src/components/email-view.tsx

@@ -1398,12 +1398,12 @@ const ComposeBox = memo(function ComposeBox({
             : (hasGenerated ? 'Edit' : 'Write')}
         </Button>
       </div>
-      <div className="flex flex-wrap gap-1.5 border-b border-border px-3 pb-2.5">
+      <div className="flex flex-wrap gap-x-1.5 gap-y-2 border-b border-border px-3 pb-2.5">
         <Button
           type="button"
           variant="outline"
           size="xs"
-          className="rounded-full"
+          className="rounded-md"
           onClick={() => { void runAi('Improve the clarity, grammar, and flow of this email while preserving its meaning.', 'rewrite') }}
           disabled={generating}
         >Improve</Button>
@@ -1413,7 +1413,7 @@ const ComposeBox = memo(function ComposeBox({
             type="button"
             variant="outline"
             size="xs"
-            className="rounded-full"
+            className="rounded-md"
             onClick={() => { void runAi(preset.instruction, 'rewrite') }}
             disabled={generating}
           >{preset.label}</Button>