feat(collab): drop conservative content-skip when collab active + applier toast (TASK-1262) (#460)

## Drop TASK-1243's content-skip when collab is active

The conservative `item = { ...updated, content: item.content }`
preservation in the SSE/sync handlers was protecting against
clobbering a user's mid-keystroke edit with a stale content
snapshot. Under collab the editor reads from Y.Doc — NOT the
content prop — so the Editor.svelte $effect's `if (ydoc) return`
gate at line 810 makes adopting `updated.content` harmless to
the live editor while keeping `item.content` fresh for
downstream consumers (UI summaries, search-index hints,
subsequent share-page renders).

For non-collab viewers (view-only, raw mode, items where
canEdit=false) the content-skip stays — those paths DO render
from item.content via the prop $effect, and adopting a stale
SSE snapshot mid-keystroke would clobber unsaved chars.

Applied to all four adoption sites:
  - SSE item_updated
  - SSE item_restored
  - syncService incremental update
  - syncService full-refresh fallback

## Applier-success toast

The applier handler (wired in TASK-1259's absorption of TASK-1262
scope) silently called setContent. Users would see their editor
change under them with no UI hint. Adds a brief
toastStore.show('External edit applied', 'info') after the
setContent succeeds; preserves the late-apply guard so toasts
only fire on actual mutations.

## Acceptance criteria

- [x] `pad item update REF --stdin < new.md` while two browser
  tabs are open: both tabs reflect the change (the applier path
  fires setContent on the longest-connected tab; ops broadcast
  to peers; SSE adoption keeps item.content fresh).
- [x] `pad item update REF --status done` (field-only): both
  tabs see the field change via SSE; no editor disruption (the
  guard `input.Content != nil` skips the applier branch
  entirely; SSE adoption updates fields atomically).
- [x] Designated client disconnects mid-flight: server retries
  next applier (TASK-1257 logic; pending follow-up TASK-1268
  for the all-applier-failed case).
- [x] Toast: "External edit applied" surfaced.

Parent: PLAN-1248

web/src/routes/[username]/[workspace]/[collection]/[slug]/+page.svelte

@@ -226,7 +226,27 @@
 						const updated = await api.items.get(reqWsSlug, reqItemSlug);
 						// Bail if the user navigated away before this resolved.
 						if (!item || item.id !== reqItemId) return;
-						item = { ...updated, content: item.content };
+						// Drop TASK-1243's conservative content-skip
+						// when collab is active (TASK-1262). Under
+						// collab the editor reads from Y.Doc, NOT
+						// from the `content` prop — Editor.svelte's
+						// `if (ydoc) return` gate at the prop $effect
+						// makes adopting updated.content harmless to
+						// the live editor while keeping item.content
+						// fresh for downstream consumers (UI summaries,
+						// search index hints, etc.).
+						//
+						// Non-collab still preserves the local content
+						// to avoid clobbering an unsaved typing burst —
+						// the saveStatus guard above already skips when
+						// a debounced save is in flight, but a user
+						// mid-keystroke with no save yet pending would
+						// still lose chars without this branch.
+						if (collabProvider) {
+							item = updated;
+						} else {
+							item = { ...updated, content: item.content };
+						}
 						const links = await api.links.list(reqWsSlug, updated.slug).catch(() => []);
 						if (!item || item.id !== reqItemId) return;
 						itemLinks = links;
@@ -239,7 +259,11 @@
 					try {
 						const updated = await api.items.get(reqWsSlug, reqItemSlug);
 						if (!item || item.id !== reqItemId) return;
-						item = { ...updated, content: item.content };
+						if (collabProvider) {
+							item = updated;
+						} else {
+							item = { ...updated, content: item.content };
+						}
 					} catch {
 						// Ignore — will catch up on next event
 					}
@@ -277,12 +301,15 @@
 				// Check if our item is in the changed set
 				const updated = result.changes.updated.find(i => i.id === reqItemId);
 				if (updated) {
-					// Merge server state without disrupting the editor
+					// Merge server state without disrupting the editor.
+					// Same collab-aware adoption rule as the SSE
+					// handler above (TASK-1262).
 					if (!item || item.id !== reqItemId) return;
-					item = {
-						...updated,
-						content: item.content
-					};
+					if (collabProvider) {
+						item = updated;
+					} else {
+						item = { ...updated, content: item.content };
+					}
 					const links = await api.links.list(reqWsSlug, updated.slug).catch(() => []);
 					if (!item || item.id !== reqItemId) return;
 					itemLinks = links;
@@ -294,10 +321,11 @@
 			try {
 				const updated = await api.items.get(reqWsSlug, reqItemSlug);
 				if (!item || item.id !== reqItemId) return;
-				item = {
-					...updated,
-					content: item.content
-				};
+				if (collabProvider) {
+					item = updated;
+				} else {
+					item = { ...updated, content: item.content };
+				}
 				const links = await api.links.list(reqWsSlug, updated.slug).catch(() => []);
 				if (!item || item.id !== reqItemId) return;
 				itemLinks = links;
@@ -497,6 +525,12 @@
 				}
 				try {
 					editorInstance.commands.setContent(markdown);
+					// Brief notification so users see WHY their editor
+					// just changed under them. The applier path is
+					// triggered by external (CLI / MCP / API) writes
+					// — a silent setContent would otherwise look like
+					// a glitch. Per TASK-1262 acceptance criteria.
+					toastStore.show('External edit applied', 'info');
 					return true;
 				} catch (err) {
 					console.warn('collab: setContent failed', err);