feat: chunked file save + export progress spinner + password-reveal + avatar size

Large file save (fixes black screen on big attachment download / export)
- Bridge.saveFile streams anything over ~1MB base64 in chunks through the
  cmd:// channel instead of stuffing the whole payload in one URL — a
  multi-MB base64 URL blew past WebView2's ~2MB navigation cap and blanked
  the document (black screen). Small payloads keep the single-shot path.
- Chunks are sent sequentially (each acked via Bridge.onFileChunkAck
  before the next) so repeated location.href assignments don't coalesce.
- Delphi accumulates chunks per reqId in a TStringBuilder
  (FFileSaveChunks), commits on file/save-commit, and shares the
  decode+dialog+write logic with the single-shot path via SaveDecodedFile.
- Chunk size 1MB → far fewer round-trips (a 20MB export dropped from ~67
  to ~27 hops).

Busy overlay + progress
- Global spinner overlay (showBusy/updateBusy/hideBusy). doExport shows it
  immediately on click — BEFORE the entry-decrypt + attachment-fetch loop
  that is the real cost — with a 0ms yield so it paints before the thread
  blocks (was appearing 3-5s late). Phases: "Reading vault… N/total" →
  "Encrypting export…" → "Preparing file… N%" (real chunk progress).
  Attachment download shows the same for files > 512KB.
- Spinner ring used an undefined --bg-elev-3 (invalid border → invisible);
  switched to --border. Fixed a second stale --bg-elev-3 use on the
  settings-search clear button hover.

Password reveal
- promptDialog gets an eye toggle in password mode, so every encrypted
  prompt (export, import, backup password, recovery code, sync password)
  can show/hide the typed value.

Avatar
- Top-right chip avatar enlarged 22px → 30px with the chip padding
  rebalanced.

Rebuild: BuildAssets + F9 (UMainForm.pas changed for the chunk handlers).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

css/style.css

@@ -741,11 +741,11 @@ input[type="range"]::-webkit-slider-thumb {
 
 .user-menu { position: relative; }
 .user-chip {
-    display: inline-flex; align-items: center; gap: 6px;
-    padding: 6px 12px;
+    display: inline-flex; align-items: center; gap: 8px;
+    padding: 5px 12px 5px 6px;
     background: var(--bg-elev);
     border: 1px solid var(--border);
-    border-radius: 16px;
+    border-radius: 20px;
     color: var(--text);
     font-size: 12px;
     transition: all var(--t-fast);
@@ -756,10 +756,10 @@ input[type="range"]::-webkit-slider-thumb {
    Falls back to a background-image when a custom picture is set. */
 .user-avatar {
     display: inline-flex; align-items: center; justify-content: center;
-    width: 22px; height: 22px;
-    margin-left: -4px;
+    width: 30px; height: 30px;
+    margin-left: 0;
     border-radius: 50%;
-    font-size: 11px; font-weight: 600;
+    font-size: 13px; font-weight: 600;
     color: #fff;
     text-transform: uppercase;
     background-size: cover; background-position: center;
@@ -2178,7 +2178,7 @@ body[data-editor-position="center"]:has(#settingsPanel.is-open)::before {
     display: none;
 }
 .settings-search-clear svg { width: 12px; height: 12px; }
-.settings-search-clear:hover { color: var(--text); background: var(--bg-elev-3); }
+.settings-search-clear:hover { color: var(--text); background: var(--bg-elev-2); }
 .settings-search-wrap.has-query .settings-search-clear { display: inline-flex; }
 /* Hidden section + per-row + no-results banner driven by JS. */
 .slideover-field.is-search-hidden,
@@ -2191,6 +2191,33 @@ body[data-editor-position="center"]:has(#settingsPanel.is-open)::before {
     display: none;
 }
 .settings-no-results.is-visible { display: block; }
+/* Global busy overlay — spinner + text for long ops (export, big saves). */
+.busy-overlay {
+    position: fixed; inset: 0;
+    z-index: 200;
+    display: flex; align-items: center; justify-content: center;
+    background: rgba(0, 0, 0, 0.5);
+    backdrop-filter: blur(3px);
+    -webkit-backdrop-filter: blur(3px);
+}
+.busy-overlay.is-hidden { display: none; }
+.busy-box {
+    display: flex; flex-direction: column; align-items: center; gap: 14px;
+    padding: 24px 32px;
+    background: var(--bg-elev);
+    border: 1px solid var(--border);
+    border-radius: var(--radius-lg);
+    box-shadow: var(--shadow-lg);
+}
+.busy-spinner {
+    width: 34px; height: 34px;
+    border: 3px solid var(--border);
+    border-top-color: var(--accent);
+    border-radius: 50%;
+    animation: busy-spin 0.7s linear infinite;
+}
+@keyframes busy-spin { to { transform: rotate(360deg); } }
+.busy-text { font-size: 13px; color: var(--text-dim); text-align: center; }
 .slideover-field-label { font-size: 11px; font-weight: 500; color: var(--text-faint); text-transform: uppercase; letter-spacing: 0.5px; margin-bottom: 6px; }
 .slideover-field-value {
     display: flex; align-items: center; gap: 6px;