feat(obsidian): persist FMG-to-note association with fmg-id in frontmatter

When a user selects or creates a note for a burg/marker, the system now
automatically links them together by adding/updating the note's frontmatter.

**How it works:**

1. **When saving a note**:
   - Adds `fmg-id: burg123` to frontmatter
   - Updates `x:` and `y:` coordinates
   - Preserves existing frontmatter fields

2. **When creating a new note**:
   - Template includes fmg-id from the start
   - Uses the proper elementId (e.g., "burg123")

3. **Next time you edit**:
   - System finds note instantly by fmg-id
   - No need to search by coordinates again
   - Permanent two-way link established

**Changes:**
- obsidian-notes-editor.js:
  - Pass elementId and coordinates to showMarkdownEditor()
  - updateFrontmatterWithFmgData() injects fmg-id on save
  - Automatically updates coordinates to keep in sync
- obsidian-bridge.js:
  - generateNoteTemplate() accepts elementId parameter
  - Uses elementId for fmg-id instead of element.id

Users no longer need to manually match notes - once linked, the
association is permanent in the note's frontmatter.

modules/io/obsidian-bridge.js

@@ -343,13 +343,13 @@ const ObsidianBridge = (() => {
   }
 
   // Generate note template for FMG element
-  function generateNoteTemplate(element, type) {
+  function generateNoteTemplate(element, type, elementId) {
     const {x, y} = element;
     const lat = pack.cells.lat?.[element.cell] || 0;
     const lon = pack.cells.lon?.[element.cell] || 0;
 
     const frontmatter = {
-      "fmg-id": element.id || `${type}${element.i}`,
+      "fmg-id": elementId || element.id || `${type}${element.i}`,
       "fmg-type": type,
       coordinates: {x, y, lat, lon},
       tags: [type],