v0.8.1:0 - delete model weights from disk via card trash icon

Each model card now shows whether its weights are present on disk
(with GB size) or not yet downloaded. When present and the model
isn't currently loaded, a trash icon appears; clicking it pops a
confirmation showing exactly how many GB will be freed and on
which Spark(s), then runs rm -rf on the HF cache directory via SSH.

Cluster-mode models are removed from both Sparks; solo-mode from
Spark 1 only. Safety rails: refuses to delete the currently-loaded
model, refuses during an in-flight swap or download, and the
catalog entry stays intact so it can be re-downloaded anytime.

Backend:
  - new image/app/disk.py: probe_disk + delete_from_disk over SSH
  - GET  /api/models/disk-status — parallel probe across all catalog models
  - DELETE /api/models/{key}/disk — guarded rm -rf, logs to connectivity events

Frontend:
  - on-disk / not-downloaded pills on every card
  - trash icon-btn in card-actions row (hidden when not on disk)
  - confirmation dialog showing per-host bytes-to-free
  - disk-status re-checked every 60s

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

image/app/static/index.html

@@ -188,6 +188,20 @@
         </form>
       </dialog>
 
+      <dialog id="disk-delete-dialog" class="modal">
+        <form method="dialog" class="modal-form">
+          <h3>Delete model weights from disk?</h3>
+          <p id="dd-summary" class="muted small"></p>
+          <ul class="muted small dd-hosts" id="dd-hosts"></ul>
+          <p class="muted small">This is reversible — you can re-download from the catalog at any time. The catalog entry stays intact.</p>
+          <p id="dd-error" class="muted small dd-error hidden"></p>
+          <div class="modal-actions">
+            <button type="button" id="dd-cancel" class="btn">Cancel</button>
+            <button type="button" id="dd-confirm" class="btn danger">Delete from disk</button>
+          </div>
+        </form>
+      </dialog>
+
       <dialog id="advanced-dialog" class="modal">
         <form method="dialog" class="modal-form" id="advanced-form">
           <h3 id="adv-title">Advanced settings</h3>