[feat](trx-client): capability-gated UI controls and filter panel (UC-05..07)

Add /set_bandwidth and /set_fir_taps HTTP endpoints to api.rs.

Add applyCapabilities(caps) function to app.js that shows/hides:
- PTT button and TX meters: capabilities.tx
- TX limit row: capabilities.tx_limit
- VFO row: capabilities.vfo_switch
- Signal meter row: capabilities.signal_meter
- Filters panel: capabilities.filter_controls

Called from render() whenever capabilities are present; runs on both
initial /status response and every SSE event.

Add a Filters panel to index.html with bandwidth slider (1..500 kHz)
and FIR taps select (16/32/64/128/256); hidden by default, revealed by
applyCapabilities when filter_controls is set. Each control dispatches
to the corresponding HTTP endpoint on change.

Sync filter state from update.filter in render() to keep slider/select
in sync with server-side DSP state.

Fix missing struct fields in test helpers across remote_client.rs,
trx-frontend-http-json/server.rs, trx-frontend-rigctl/server.rs, and
trx-core controller tests (handlers.rs, machine.rs).

Update aidocs/UI-CAPS.md: all tasks UC-01..UC-09 marked [x].

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -133,6 +133,26 @@
           <button id="tx-limit-btn" type="button">Set</button>
         </div>
       </div>
+      <div id="filters-panel" style="display:none;">
+        <div class="label"><span>Filters</span></div>
+        <div class="inline" style="gap: 0.8rem; flex-wrap: wrap; align-items: center;">
+          <label style="display:flex; align-items:center; gap:0.4rem;">
+            <span style="color:var(--text-muted); font-size:0.85rem; white-space:nowrap;">BW</span>
+            <input type="range" id="bw-slider" min="1000" max="500000" step="1000" value="3000" style="width:120px;" />
+            <span id="bw-value" style="min-width:4rem; font-size:0.9rem;">3.0 kHz</span>
+          </label>
+          <label style="display:flex; align-items:center; gap:0.4rem;">
+            <span style="color:var(--text-muted); font-size:0.85rem; white-space:nowrap;">FIR taps</span>
+            <select id="fir-taps-select" class="status-input" style="width:auto; height:var(--control-height);">
+              <option value="16">16</option>
+              <option value="32">32</option>
+              <option value="64" selected>64</option>
+              <option value="128">128</option>
+              <option value="256">256</option>
+            </select>
+          </label>
+        </div>
+      </div>
     </div>
     <div class="full-row label-below-row" id="audio-row">
       <div class="label"><span>Audio</span></div>