chore(deps): bump @pierre/diffs to 1.2.12 (stage 1 of 2) (#1188)

Bump the exact pin from 1.2.8 to 1.2.12 in all six package.json files
(root, packages/ui, packages/server, packages/review-editor,
apps/review, apps/pi-extension) and resolve the lockfile.

1.2.12 pulls in @pierre/theme 1.1.0 (minor, transitive-only; we have no
direct theme dependency) and a new transitive @pierre/theming 0.0.2.
Stage 2 (1.3.x, next week once aged) carries the theme major and the
hover pipeline rework.

Verified: typecheck, full bun test, CI DOM set, all four builds, SSR
parity at 1.2.8 vs 1.2.12 (structure identical except the intentional
tabindex removal from the 1.2.12 focus fix), and a fresh registry
install of the pi-extension tarball resolving a working 1.2.12
(guarding against the 1.2.9-era broken-tarball failure mode, #880).

package.json

@@ -39,7 +39,7 @@
   "dependencies": {
     "@anthropic-ai/claude-agent-sdk": "^0.2.92",
     "@opencode-ai/sdk": "^1.3.0",
-    "@pierre/diffs": "1.2.8",
+    "@pierre/diffs": "1.2.12",
     "diff": "^8.0.4",
     "dockview-react": "^5.2.0",
     "dompurify": "^3.3.3",