chore: legacy-tree notices — PR bot, sentinel READMEs, PR template

Two contributors in one week sent solid PRs against the retired
Tauri/Svelte tree (#561, #564) because nothing in the repo says it is
dead. Three zero-risk notices, no code touched: a pull_request_target
bot that leaves one sticky comment when a PR touches src//src-tauri/
(target on purpose — fork PRs cannot be commented under plain
pull_request; the PR code is never checked out), sentinel READMEs at
both tree roots (what GitHub renders when browsing), and a PR template
checkbox. The tree deletion + local reference worktree is the 2.0.2
follow-up (owner hold: no dependency risk allowed before 2.0.1).

cc

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,10 @@
+## Summary
+
+<!-- What does this PR do? -->
+
+## Checklist
+
+- [ ] My change does **not** touch `src/` or `src-tauri/` — those trees are the
+      retired pre-2.0 frontend, kept only as a porting reference. The live app
+      is the Slint UI under `crates/`. (If your fix targets the old tree, open
+      an issue instead and we'll help you port it.)

.github/workflows/legacy-tree-notice.yml

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+name: legacy-tree-notice
+
+# The Tauri/Svelte frontend (src/, src-tauri/) is RETIRED — kept in-tree only
+# as the 1:1 port reference until the 2.x cleanup removes it. Contributors
+# keep sending PRs against it (#561, #564), so this bot leaves one sticky
+# comment when a PR touches those paths. pull_request_target ON PURPOSE:
+# fork PRs get a read-only token under plain pull_request and could not be
+# commented on. Safe: the PR's code is NEVER checked out or executed here —
+# we only read the changed-file list and post a comment.
+
+on:
+  pull_request_target:
+    types: [opened, synchronize]
+
+permissions:
+  pull-requests: write
+
+jobs:
+  notice:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Comment when the PR touches the legacy tree
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const files = await github.paginate(github.rest.pulls.listFiles, {
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              pull_number: context.payload.pull_request.number,
+              per_page: 100,
+            });
+            const legacy = files.filter(f =>
+              f.filename.startsWith('src/') || f.filename.startsWith('src-tauri/'));
+            if (legacy.length === 0) return;
+            const marker = '<!-- legacy-tree-notice -->';
+            const comments = await github.paginate(github.rest.issues.listComments, {
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: context.payload.pull_request.number,
+              per_page: 100,
+            });
+            if (comments.some(c => c.body && c.body.includes(marker))) return;
+            const body = [
+              marker,
+              'Hi! Thanks a lot for contributing.',
+              '',
+              'Heads up: this change touches `src/` / `src-tauri/`, which is the **retired Tauri/Svelte frontend**. Since 2.0 the app that actually ships is the native Slint UI under `crates/` — the old tree stays in the repo only as a porting reference and will be deleted soon.',
+              '',
+              'Your analysis is still valuable! If the bug/feature also applies to the live app, we are happy to help port it — recent examples: #561 (Dutch translation) and #564 (favorite sync) were both re-implemented in `crates/` with credit to their authors.',
+            ].join('\n');
+            await github.rest.issues.createComment({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: context.payload.pull_request.number,
+              body,
+            });

src-tauri/README.md

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+# ⚠️ RETIRED — legacy Tauri backend (do not submit PRs here)
+
+This tree is the pre-2.0 Tauri backend. **It no longer ships**: since QBZ 2.0
+the app is a single native Rust process with a Slint UI living under
+[`crates/`](../crates/). This code remains in the repository only as a
+porting reference and will be removed in an upcoming cleanup release.
+
+If you found a bug here, it very likely exists (or was already fixed) in the
+live app — please target `crates/` instead, or open an issue and we will help
+you port the change.

src/README.md

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+# ⚠️ RETIRED — legacy Svelte frontend (do not submit PRs here)
+
+This tree is the pre-2.0 Svelte UI. **It no longer ships**: since QBZ 2.0 the
+app is a single native Rust process with a Slint UI living under
+[`crates/`](../crates/). This code remains in the repository only as a
+porting reference and will be removed in an upcoming cleanup release.
+
+If you found a bug here, it very likely exists (or was already fixed) in the
+live app — please target `crates/` instead, or open an issue and we will help
+you port the change.