feat(v2): implement session persistence, file watcher, and markdown viewer

Phase 4 complete (MVP ship):
- SessionDb (rusqlite, WAL mode): sessions + layout_state tables, CRUD
- FileWatcherManager (notify v6): watch files, emit Tauri change events
- MarkdownPane: marked.js rendering with Catppuccin styles, live reload
- Layout store wired to persistence (addPane/removePane/setPreset persist)
- restoreFromDb() on startup restores panes in layout order
- Sidebar "M" button opens file picker for markdown files
- New adapters: session-bridge.ts, file-bridge.ts
- Deps: rusqlite (bundled), dirs 5, notify 6, marked

v2/src/lib/components/Markdown/MarkdownPane.svelte

@@ -0,0 +1,196 @@
+<script lang="ts">
+  import { onMount, onDestroy } from 'svelte';
+  import { marked } from 'marked';
+  import { watchFile, unwatchFile, onFileChanged, type FileChangedPayload } from '../../adapters/file-bridge';
+
+  interface Props {
+    filePath: string;
+    paneId: string;
+    onExit?: () => void;
+  }
+
+  let { filePath, paneId, onExit }: Props = $props();
+
+  let renderedHtml = $state('');
+  let error = $state('');
+  let unlisten: (() => void) | undefined;
+
+  function renderMarkdown(source: string): void {
+    try {
+      renderedHtml = marked.parse(source, { async: false }) as string;
+      error = '';
+    } catch (e) {
+      error = `Render error: ${e}`;
+    }
+  }
+
+  onMount(async () => {
+    try {
+      const content = await watchFile(paneId, filePath);
+      renderMarkdown(content);
+
+      unlisten = await onFileChanged((payload: FileChangedPayload) => {
+        if (payload.pane_id === paneId) {
+          renderMarkdown(payload.content);
+        }
+      });
+    } catch (e) {
+      error = `Failed to open file: ${e}`;
+    }
+  });
+
+  onDestroy(() => {
+    unlisten?.();
+    unwatchFile(paneId).catch(() => {});
+  });
+</script>
+
+<div class="markdown-pane">
+  {#if error}
+    <div class="error">{error}</div>
+  {:else}
+    <div class="markdown-body">
+      {@html renderedHtml}
+    </div>
+  {/if}
+  <div class="file-path">{filePath}</div>
+</div>
+
+<style>
+  .markdown-pane {
+    display: flex;
+    flex-direction: column;
+    height: 100%;
+    background: var(--bg-primary);
+    color: var(--text-primary);
+  }
+
+  .markdown-body {
+    flex: 1;
+    overflow-y: auto;
+    padding: 16px 20px;
+    font-size: 14px;
+    line-height: 1.6;
+  }
+
+  .markdown-body :global(h1) {
+    font-size: 1.6em;
+    font-weight: 700;
+    margin: 0.8em 0 0.4em;
+    color: var(--ctp-lavender);
+    border-bottom: 1px solid var(--border);
+    padding-bottom: 0.3em;
+  }
+
+  .markdown-body :global(h2) {
+    font-size: 1.3em;
+    font-weight: 600;
+    margin: 0.7em 0 0.3em;
+    color: var(--ctp-blue);
+  }
+
+  .markdown-body :global(h3) {
+    font-size: 1.1em;
+    font-weight: 600;
+    margin: 0.6em 0 0.3em;
+    color: var(--ctp-sapphire);
+  }
+
+  .markdown-body :global(p) {
+    margin: 0.5em 0;
+  }
+
+  .markdown-body :global(code) {
+    background: var(--bg-surface);
+    padding: 1px 5px;
+    border-radius: 3px;
+    font-family: var(--font-mono);
+    font-size: 0.9em;
+    color: var(--ctp-green);
+  }
+
+  .markdown-body :global(pre) {
+    background: var(--bg-surface);
+    padding: 12px 14px;
+    border-radius: var(--border-radius);
+    overflow-x: auto;
+    font-size: 12px;
+    line-height: 1.5;
+    margin: 0.6em 0;
+  }
+
+  .markdown-body :global(pre code) {
+    background: none;
+    padding: 0;
+    color: var(--text-primary);
+  }
+
+  .markdown-body :global(blockquote) {
+    border-left: 3px solid var(--ctp-mauve);
+    margin: 0.5em 0;
+    padding: 4px 12px;
+    color: var(--text-secondary);
+  }
+
+  .markdown-body :global(ul), .markdown-body :global(ol) {
+    padding-left: 24px;
+    margin: 0.4em 0;
+  }
+
+  .markdown-body :global(li) {
+    margin: 0.2em 0;
+  }
+
+  .markdown-body :global(a) {
+    color: var(--ctp-blue);
+    text-decoration: none;
+  }
+
+  .markdown-body :global(a:hover) {
+    text-decoration: underline;
+  }
+
+  .markdown-body :global(table) {
+    border-collapse: collapse;
+    width: 100%;
+    margin: 0.5em 0;
+    font-size: 13px;
+  }
+
+  .markdown-body :global(th), .markdown-body :global(td) {
+    border: 1px solid var(--border);
+    padding: 6px 10px;
+    text-align: left;
+  }
+
+  .markdown-body :global(th) {
+    background: var(--bg-surface);
+    font-weight: 600;
+  }
+
+  .markdown-body :global(hr) {
+    border: none;
+    border-top: 1px solid var(--border);
+    margin: 1em 0;
+  }
+
+  .markdown-body :global(img) {
+    max-width: 100%;
+    border-radius: var(--border-radius);
+  }
+
+  .file-path {
+    border-top: 1px solid var(--border);
+    padding: 4px 12px;
+    font-size: 10px;
+    font-family: var(--font-mono);
+    color: var(--text-muted);
+    flex-shrink: 0;
+  }
+
+  .error {
+    color: var(--ctp-red);
+    padding: 16px;
+    font-size: 13px;
+  }
+</style>