[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-rigctl/src/server.rs

@@ -633,6 +633,11 @@ mod tests {
                     rit: false,
                     rpt: false,
                     split: false,
+                    tx: true,
+                    tx_limit: true,
+                    vfo_switch: true,
+                    filter_controls: false,
+                    signal_meter: true,
                 },
                 access: RigAccessMethod::Tcp {
                     addr: "127.0.0.1:4532".to_string(),
@@ -668,6 +673,7 @@ mod tests {
             cw_auto: false,
             cw_wpm: 0,
             cw_tone_hz: 0,
+            filter: None,
         }
     }