refactor: drive attachment sending state through the chat activity dock

Replace the composer-local "Sending…" indicator with a single, consistent
state surfaced by the existing chat activity dock. The session controller
now owns the full attachment send lifecycle (including the folder-mode
upload + reference rewrite) and toggles an isSendingPrompt flag around it,
which the chat dock renders as "Sending your message…" until the real
server activity supersedes it.

This covers the pre-receipt dead zone (upload, server-side image resize,
first-session open) with one indicator instead of two, and keeps plain
text sends fire-and-forget.

src/client/src/components/shared.ts

@@ -459,7 +459,6 @@ export const promptEditorStyles = css`
   .markdown-editor .cm-focused { outline: none; }
   .shell-mode textarea, .shell-mode .markdown-editor .cm-editor { border-color: var(--pi-success); box-shadow: 0 0 0 1px var(--pi-success-ring); }
   .mode-hint { position: absolute; right: 8px; bottom: 8px; max-width: calc(100% - 16px); border: 1px solid var(--pi-success-border); border-radius: 999px; background: var(--pi-success-surface); color: var(--pi-success); padding: 2px 8px; font-size: 12px; pointer-events: none; }
-  .sending-hint { border-color: var(--pi-accent-border); background: var(--pi-selection-bg); color: var(--pi-accent); }
   .attachments { display: flex; flex-wrap: wrap; align-items: center; gap: 8px; margin-top: 8px; }
   .attachment-chip { position: relative; width: 56px; height: 56px; border: 1px solid var(--pi-border); border-radius: 8px; overflow: hidden; background: var(--pi-bg); }
   .attachment-chip img { width: 100%; height: 100%; object-fit: cover; display: block; }