Add Tiptap inline-preview editor, layout improvements, and README

Replace plain textarea with Tiptap (ProseMirror) WYSIWYG editor for notes
and tasks. Headings, bold, lists, and code blocks now render inline while
editing. Tags and wikilinks get visual highlighting via decoration plugins,
and autocomplete uses @tiptap/suggestion with heading disambiguation.

Layout: pin navbar with app-shell flex column (100dvh), sticky editor
toolbar above scrolling content, AI Assist panel fixed at bottom 1/3 with
full-height section selector and prompt. Increase max-width to 960px
across all views. Hide assist controls during streaming/review.

Other fixes: markdown paste handling, auto-syncing assist sections via
body watcher, trailing newline on AI accept, ChatView height fix.

Add README.md describing the app, usage guide, and technical overview.
Update summary.md with Phase 5.2 roadmap and current status.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/fabledassistant/services/assist.py

@@ -0,0 +1,35 @@
+MAX_BODY_CHARS = 3000
+
+
+def build_assist_messages(
+    body: str, target_section: str, instruction: str
+) -> list[dict]:
+    """Build Ollama messages for section-level assist.
+
+    The full note body (truncated) is provided as read-only context in the
+    system prompt.  The target section + user instruction go in the user
+    message.  The model outputs only the replacement for the target section.
+    """
+    truncated_body = body[:MAX_BODY_CHARS]
+    if len(body) > MAX_BODY_CHARS:
+        truncated_body += "\n... (truncated)"
+
+    system_content = (
+        "You are an AI writing assistant integrated into a note-taking app. "
+        "The user is editing a document. The full document is shown below for context.\n\n"
+        f"--- Full Document ---\n{truncated_body}\n--- End Document ---\n\n"
+        "The user will give you a specific section of the document and an instruction. "
+        "Output ONLY the replacement text for that section. "
+        "Do not include other sections, explanatory text, or markdown code fences around the output. "
+        "Match the document's existing tone and style."
+    )
+
+    user_content = (
+        f"--- Target Section ---\n{target_section}\n--- End Target Section ---\n\n"
+        f"Instruction: {instruction}"
+    )
+
+    return [
+        {"role": "system", "content": system_content},
+        {"role": "user", "content": user_content},
+    ]