fix(ui): label the symbol-only notes-toolbar buttons (T7.5, FR-UX-4)

Bold/Italic/Bullet-list/Checkbox buttons rendered a bare glyph with
only a title tooltip; add matching aria-label so a screen reader
announces the action rather than the raw Unicode character name.

src/lib/views/TranscriptNotes.svelte

@@ -128,12 +128,24 @@
       </div>
       <div class="pane notes">
         <div class="toolbar" role="toolbar" aria-label="Notes formatting">
-          <button onclick={() => wrapSelection("**")} title="Bold"><b>B</b></button>
-          <button onclick={() => wrapSelection("_")} title="Italic"><i>I</i></button>
+          <button onclick={() => wrapSelection("**")} title="Bold" aria-label="Bold"
+            ><b>B</b></button
+          >
+          <button onclick={() => wrapSelection("_")} title="Italic" aria-label="Italic"
+            ><i>I</i></button
+          >
           <button onclick={() => insertLinePrefix("# ")} title="Heading 1">H1</button>
           <button onclick={() => insertLinePrefix("## ")} title="Heading 2">H2</button>
-          <button onclick={() => insertLinePrefix("- ")} title="Bullet list">•</button>
-          <button onclick={() => insertLinePrefix("- [ ] ")} title="Checkbox">☑</button>
+          <button
+            onclick={() => insertLinePrefix("- ")}
+            title="Bullet list"
+            aria-label="Bullet list">•</button
+          >
+          <button
+            onclick={() => insertLinePrefix("- [ ] ")}
+            title="Checkbox"
+            aria-label="Checkbox list item">☑</button
+          >
           <span class="spacer"></span>
           <button onclick={exportMd} title="Export notes as .md">Export .md</button>
           <button onclick={exportBundle} title="Export audio + transcript + notes to a folder">