Show the complete mailbox; sync_limit now caps body prefetch only

The local store was capped at sync_limit, so IMAP only ever listed the
newest N messages — a search in Thunderbird (the only search UI we have)
silently missed everything older. Now the syncer lists the *full* mailbox
metadata for every folder, and sync_limit governs only how many recent
message bodies are pre-warmed offline. Bodies outside that window are
fetched on demand the first time a client opens the message.

A one-time full-metadata sync (marker full_metadata_synced_v1) completes
the mailbox view on first launch after upgrade.

Crucially, an empty body is now stored as rfc2822 = None rather than a
rendered "(No body available)" placeholder: the placeholder looked like a
real body to the IMAP layer and suppressed the on-demand fetch. CachedMail
gains body_loaded to track whether the body (not just the headers) is final.

Validated live on a 19,322-message INBOX (26,965 mails total across
folders): full listing, on-demand body fetch (~0.1-0.4s), in-memory cache
on re-fetch.

ui/src/components/ConfigPanel.tsx

@@ -82,18 +82,23 @@ export function ConfigPanel({ config, status, loading, onSave, onRestart }: Prop
         />
       </div>
       <div className="form-group">
-        <label>Mail to sync</label>
+        <label>Offline message bodies</label>
+        <small className="field-hint">
+          Your whole mailbox is always listed and searchable by subject, sender
+          and date. This only sets how many recent message <em>bodies</em> are
+          kept ready offline — older ones load on demand when you open them.
+        </small>
         <label className="checkbox-field">
           <input
             type="checkbox"
             checked={fetchAll}
             onChange={(e) => setFetchAll(e.target.checked)}
           />
-          <span>Fetch all mail (entire account, kept locally)</span>
+          <span>Keep every message body offline (full local copy)</span>
         </label>
         {fetchAll ? (
           <small className="field-hint">
-            Downloads every mail from the start — can be slow on large accounts.
+            Downloads every body — slow + uses the most disk on large accounts.
           </small>
         ) : (
           <input
@@ -101,7 +106,7 @@ export function ConfigPanel({ config, status, loading, onSave, onRestart }: Prop
             min={1}
             value={syncLimit}
             onChange={(e) => setSyncLimit(Math.max(1, Number(e.target.value)))}
-            placeholder="Max mails per folder"
+            placeholder="Bodies to keep offline (most recent)"
           />
         )}
       </div>