feat: add Opus 32k/48k/64k studio quality tiers

Adds three new codec IDs (Opus32k=6, Opus48k=7, Opus64k=8) and
corresponding STUDIO_32K, STUDIO_48K, STUDIO_64K quality profiles.
All use 20ms frames with minimal FEC (10%) for maximum quality on
good networks.

Updated across: wire protocol (codec_id.rs), encoder/decoder
(opus_enc/dec.rs), adaptive codec switch (call.rs), CLI
(--profile studio-64k), desktop engine + UI slider (8 quality
levels from Studio 64k green to Codec2 1.2k red).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/index.html

@@ -96,13 +96,16 @@
                 <span class="setting-label">QUALITY</span>
                 <span id="s-quality-label" class="quality-label">Auto</span>
               </div>
-              <input id="s-quality" type="range" min="0" max="4" step="1" value="0" class="quality-slider" />
+              <input id="s-quality" type="range" min="0" max="7" step="1" value="3" class="quality-slider" />
               <div class="quality-ticks">
+                <span>64k</span>
+                <span>48k</span>
+                <span>32k</span>
                 <span>Auto</span>
-                <span>Opus 24k</span>
-                <span>Opus 6k</span>
-                <span>C2 3.2k</span>
-                <span>C2 1.2k</span>
+                <span>24k</span>
+                <span>6k</span>
+                <span>C2</span>
+                <span>1.2k</span>
               </div>
             </div>
             <label class="checkbox">