feat(messages): word diff of the last edit on "(edited)" hover (#144)

Hovering or focusing "(edited)" shows a tooltip with only the most recent
edit as a word diff — removed words struck, added words bold — plus a
+N/−N summary. Clicking still opens the full history viewer. On touch, a
long-press on the label shows the same diff as a popout (a plain tap opens
the viewer; the message's own long-press action sheet is not triggered).

utils/wordDiff.ts is a unit-tested LCS over words that ignores whitespace-
only changes and gives up past 400 words. Only plain-text bodies are
diffed; formatted edits fall back to the viewer.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/message/content/EditDiffContext.ts

@@ -0,0 +1,6 @@
+import { createContext, useContext } from 'react';
+import { DiffToken } from '../../../utils/wordDiff';
+
+/** [Gitea #144] Word diff of the message's last edit, for the "(edited)" hover. */
+export const EditDiffContext = createContext<DiffToken[] | undefined>(undefined);
+export const useEditDiff = () => useContext(EditDiffContext);