feat(words): use Editable for ALT/CAPTION in image drawer panel

User-suggested polish: replace the read-only `<p>` previews for the
image alt and caption fields with the canonical Editable component.
Same inline-edit shape used in the dedicated Editable demo — preview
text with a small "Edit" trigger that swaps to an input + Save/Cancel
on activation. Closer to the Notion/Figma/Linear interaction the user
expects out of a sidebar property panel.

Wiring:
- `imageBlockAtCaret` now returns `{ index, block }` so the panel
  can dispatch `applyCommand({ type:'updateBlock', blockIndex, patch })`
  with the right index instead of re-walking the selection.
- ALT: `onValueCommit={(next) => s.applyCommand({ type:'updateBlock',
  blockIndex, patch: { alt: next } })}`. `placeholder` "Describe the
  image…" surfaces when alt is empty.
- CAPTION: same shape; `patch: { caption: next || undefined }` so
  clearing the field drops the field entirely from the block.
- `size="sm" variant="ghost" color="primary"` chosen to fit the
  drawer's compact density without competing with the image preview.
- Status (pending/error) stays as the existing readonly tag — not
  user-editable.

Also: add `selectedBlockIndex: number | undefined` to
`WordsProviderSnippetProps`. The provider was already emitting it
(consumed by `imageBlockAtCaret` and the image float bar) but the
type definition didn't list it.

Verified in browser: clicking the image enters image mode in the
drawer; clicking ALT's Edit trigger reveals the input; typing a new
value and clicking Save propagated end-to-end — `<img alt="…">` in
the editor and the preview both updated. Same flow for caption.

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

src/uix/eidos/components/words/words-drawer.svelte

@@ -13,6 +13,7 @@
 	 * just paints them.
 	 */
 	import { ChevronRight, ChevronLeft, ChevronDown, ChevronUp } from '$uix/eidos/components/icon';
+	import { Editable } from '$uix/eidos/components/editable';
 	import * as Words from '$soma/components/words';
 	import type {
 		DrawerSnippetProps,
@@ -65,10 +66,13 @@
 
 	function imageBlockAtCaret(s: ProviderSnippetProps):
 		| {
+				readonly index: number;
+				readonly block: {
 					readonly src: string;
 					readonly alt?: string;
 					readonly caption?: string;
 					readonly status?: 'pending' | 'error';
+				};
 		  }
 		| undefined {
 		// Atomic block selection wins (image is contenteditable=false so
@@ -81,7 +85,7 @@
 		if (blockIndex === undefined) return undefined;
 		const block = s.document.children[blockIndex];
 		if (!block || block.type !== 'image') return undefined;
-		return block;
+		return { index: blockIndex, block };
 	}
 
 	const CODE_LANGUAGES = [
@@ -545,18 +549,64 @@
 {/snippet}
 
 {#snippet imagePanel(s: ProviderSnippetProps)}
-	{@const block = imageBlockAtCaret(s)}
-	{#if block}
+	{@const entry = imageBlockAtCaret(s)}
+	{#if entry}
+		{@const block = entry.block}
+		{@const blockIndex = entry.index}
 		<div data-words-drawer-section-subtitle>Source</div>
 		<p data-words-drawer-readonly-text>{block.src}</p>
 		<div data-words-drawer-section-subtitle>Alt</div>
-		<p data-words-drawer-readonly-text data-empty={!block.alt}>
-			{block.alt || '(missing)'}
-		</p>
-		{#if block.caption}
+		<Editable
+			value={block.alt ?? ''}
+			placeholder="Describe the image…"
+			activationMode="click"
+			submitMode="both"
+			size="sm"
+			variant="ghost"
+			color="primary"
+			onValueCommit={(next) =>
+				s.applyCommand({
+					type: 'updateBlock',
+					blockIndex,
+					patch: { alt: next }
+				})}
+		>
+			<Editable.Area>
+				<Editable.Preview />
+				<Editable.Input />
+			</Editable.Area>
+			<Editable.Control>
+				<Editable.EditTrigger>Edit</Editable.EditTrigger>
+				<Editable.SubmitTrigger>Save</Editable.SubmitTrigger>
+				<Editable.CancelTrigger>Cancel</Editable.CancelTrigger>
+			</Editable.Control>
+		</Editable>
 		<div data-words-drawer-section-subtitle>Caption</div>
-			<p data-words-drawer-readonly-text>{block.caption}</p>
-		{/if}
+		<Editable
+			value={block.caption ?? ''}
+			placeholder="Optional caption shown below the image"
+			activationMode="click"
+			submitMode="both"
+			size="sm"
+			variant="ghost"
+			color="primary"
+			onValueCommit={(next) =>
+				s.applyCommand({
+					type: 'updateBlock',
+					blockIndex,
+					patch: { caption: next || undefined }
+				})}
+		>
+			<Editable.Area>
+				<Editable.Preview />
+				<Editable.Input />
+			</Editable.Area>
+			<Editable.Control>
+				<Editable.EditTrigger>Edit</Editable.EditTrigger>
+				<Editable.SubmitTrigger>Save</Editable.SubmitTrigger>
+				<Editable.CancelTrigger>Cancel</Editable.CancelTrigger>
+			</Editable.Control>
+		</Editable>
 		{#if block.status}
 			<div data-words-drawer-section-subtitle>Status</div>
 			<p data-words-drawer-readonly-text data-tone={block.status}>

src/uix/soma/components/words/types.ts

@@ -100,6 +100,14 @@ export type WordsCommandName =
 export type WordsProviderSnippetProps = {
 	readonly document: WordsDocument;
 	readonly selection: WordsSelection | null;
+	/**
+	 * Index of an atomic block currently selected via `selectAtomicBlock`
+	 * (e.g. click on an image). Coexists with `selection` but takes
+	 * priority for block-scoped UIs (drawer image panel, float bar)
+	 * because atomic blocks are `contenteditable=false` and the text
+	 * selection rarely points at them.
+	 */
+	readonly selectedBlockIndex: number | undefined;
 	readonly plainText: string;
 	readonly isEmpty: boolean;
 	readonly isFocused: boolean;