feat: @agor/stores package (3 stores) + 58 BackendAdapter tests

@agor/stores:
- theme.svelte.ts, notifications.svelte.ts, health.svelte.ts extracted
- Original files replaced with re-exports (zero consumer changes needed)
- pnpm workspace + Vite/tsconfig aliases configured

BackendAdapter tests (58 new):
- backend-adapter.test.ts: 9 tests (lifecycle, singleton, testing seam)
- tauri-adapter.test.ts: 28 tests (invoke mapping, command names, params)
- electrobun-adapter.test.ts: 21 tests (RPC names, capabilities, stubs)

Total: 523 tests passing (was 465, +58)

ui-electrobun/src/mainview/FileBrowser.svelte

@@ -170,6 +170,11 @@
       fileEncoding = result.encoding;
       fileSize = result.size;
       editorContent = fileContent;
+      // Feature 2: Record mtime at read time
+      try {
+        const stat = await appRpc.request["files.stat"]({ path: filePath });
+        if (token === fileRequestToken && !stat.error) readMtimeMs = stat.mtimeMs;
+      } catch { /* non-critical */ }
     } catch (err) {
       if (token !== fileRequestToken) return;
       fileError = err instanceof Error ? err.message : String(err);
@@ -178,9 +183,27 @@
     }
   }
 
-  /** Save current file. */
+  /** Save current file. Feature 2: Check mtime before write for conflict detection. */
   async function saveFile() {
     if (!selectedFile || !isDirty) return;
+    try {
+      // Feature 2: Check if file was modified externally since we read it
+      if (readMtimeMs > 0) {
+        const stat = await appRpc.request["files.stat"]({ path: selectedFile });
+        if (!stat.error && stat.mtimeMs > readMtimeMs) {
+          showConflictDialog = true;
+          return;
+        }
+      }
+      await doSave();
+    } catch (err) {
+      console.error('[files.write]', err);
+    }
+  }
+
+  /** Force-save, bypassing conflict check. */
+  async function doSave() {
+    if (!selectedFile) return;
     try {
       const result = await appRpc.request["files.write"]({
         path: selectedFile,
@@ -189,6 +212,10 @@
       if (result.ok) {
         isDirty = false;
         fileContent = editorContent;
+        showConflictDialog = false;
+        // Update mtime after successful save
+        const stat = await appRpc.request["files.stat"]({ path: selectedFile });
+        if (!stat.error) readMtimeMs = stat.mtimeMs;
       } else if (result.error) {
         console.error('[files.write]', result.error);
       }
@@ -197,6 +224,17 @@
     }
   }
 
+  /** Reload file from disk (discard local changes). */
+  async function reloadFile() {
+    showConflictDialog = false;
+    if (selectedFile) {
+      isDirty = false;
+      const saved = selectedFile;
+      selectedFile = null;
+      await selectFile(saved);
+    }
+  }
+
   function onEditorChange(newContent: string) {
     editorContent = newContent;
     isDirty = newContent !== fileContent;
@@ -282,6 +320,21 @@
     {@render renderEntries(cwd, 0)}
   </div>
 
+  <!-- Feature 2: Conflict dialog -->
+  {#if showConflictDialog}
+    <div class="conflict-overlay">
+      <div class="conflict-dialog">
+        <p class="conflict-title">File modified externally</p>
+        <p class="conflict-desc">This file was changed on disk since you opened it.</p>
+        <div class="conflict-actions">
+          <button class="conflict-btn overwrite" onclick={doSave}>Overwrite</button>
+          <button class="conflict-btn reload" onclick={reloadFile}>Reload</button>
+          <button class="conflict-btn cancel" onclick={() => showConflictDialog = false}>Cancel</button>
+        </div>
+      </div>
+    </div>
+  {/if}
+
   <!-- Viewer panel -->
   <div class="fb-viewer">
     {#if !selectedFile}
@@ -346,6 +399,7 @@
     height: 100%;
     overflow: hidden;
     font-size: 0.8125rem;
+    position: relative;
   }
 
   /* ── Tree panel ── */
@@ -521,4 +575,58 @@
     object-fit: contain;
     border-radius: 0.25rem;
   }
+
+  /* Feature 2: Conflict dialog */
+  .conflict-overlay {
+    position: absolute;
+    inset: 0;
+    z-index: 50;
+    background: color-mix(in srgb, var(--ctp-crust) 70%, transparent);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+  }
+
+  .conflict-dialog {
+    background: var(--ctp-surface0);
+    border: 1px solid var(--ctp-surface1);
+    border-radius: 0.5rem;
+    padding: 1rem 1.25rem;
+    max-width: 20rem;
+  }
+
+  .conflict-title {
+    margin: 0 0 0.25rem;
+    font-size: 0.875rem;
+    font-weight: 600;
+    color: var(--ctp-peach);
+  }
+
+  .conflict-desc {
+    margin: 0 0 0.75rem;
+    font-size: 0.75rem;
+    color: var(--ctp-subtext0);
+  }
+
+  .conflict-actions {
+    display: flex;
+    gap: 0.375rem;
+  }
+
+  .conflict-btn {
+    flex: 1;
+    padding: 0.25rem 0.5rem;
+    border-radius: 0.25rem;
+    border: 1px solid var(--ctp-surface1);
+    background: var(--ctp-surface0);
+    color: var(--ctp-text);
+    font-family: var(--ui-font-family);
+    font-size: 0.6875rem;
+    font-weight: 600;
+    cursor: pointer;
+  }
+
+  .conflict-btn.overwrite { border-color: var(--ctp-red); color: var(--ctp-red); }
+  .conflict-btn.reload { border-color: var(--ctp-blue); color: var(--ctp-blue); }
+  .conflict-btn:hover { background: var(--ctp-surface1); }
 </style>