feat(ui): URL-shaped keyboard + lowercase enforcement for server host

The server-host TextField on the connect form accepts a hostname or
hostname:port pair (e.g. kr.teamspeak.app:9987). Two improvements:

  1. keyboardType: TextInputType.url surfaces '.', '/', ':' on the
     primary on-screen keyboard plane so the user does not have to
     switch to the symbols pane mid-address. Matches iOS Safari's
     URL bar.

  2. textCapitalization.none + autocorrect/enableSuggestions=false
     prevents iOS from auto-capitalising the first letter or
     'correcting' 'kr.teamspeak.app' to something else.

  3. inputFormatters belt-and-braces:
       * deny whitespace (handles tab-indented paste)
       * lowercase pipeline (handles uppercase paste)

  4. Visual affordances: prefix dns icon + 'host[:port]' hint.

Nickname field intentionally unchanged \u2014 may contain unicode,
mixed case, spaces.

flutter build ios --release --no-codesign: 30.7 s, Runner.app
30.0 MB. flutter analyze clean (6 pre-existing deprecation
warnings on Radio.groupValue/onChanged).

apps/chanora_flutter/lib/main.dart

@@ -1294,11 +1294,42 @@ class _ConnectFormState extends State<_ConnectForm> {
             controller: widget.hostCtl,
             focusNode: _hostFocus,
             onTap: () => _hostFocus.requestFocus(),
+            // Server addresses are URL-shaped: hostname or
+            // hostname:port, all-lowercase ASCII, never user-
+            // friendly prose. Configure the on-screen keyboard
+            // accordingly:
+            //   * keyboardType: .url        \u2014 surfaces ".", "/",
+            //     ":" on the primary keyboard plane so the user
+            //     doesn't have to switch to the symbols pane to
+            //     type kr.teamspeak.app:9987.
+            //   * textInputAction: .next    \u2014 return key advances
+            //     to the nickname field.
+            //   * autocorrect / enableSuggestions: false  \u2014 iOS
+            //     should not autocorrect 'kr.teamspeak.app' to
+            //     'kr.teamspeak.lap' or suggest 'KR' in caps.
+            //   * textCapitalization: .none \u2014 don't capitalise the
+            //     first letter the way iOS does for sentences.
+            //   * inputFormatters: deny whitespace and uppercase\u2014
+            //     belt-and-braces in case the user pastes from a
+            //     formatted source (e.g. tab-indented copy).
+            keyboardType: TextInputType.url,
+            textCapitalization: TextCapitalization.none,
             textInputAction: TextInputAction.next,
             autocorrect: false,
             enableSuggestions: false,
+            inputFormatters: [
+              FilteringTextInputFormatter.deny(RegExp(r'\s')),
+              TextInputFormatter.withFunction(
+                (oldValue, newValue) => newValue.copyWith(
+                  text: newValue.text.toLowerCase(),
+                  selection: newValue.selection,
+                ),
+              ),
+            ],
             decoration: InputDecoration(
               labelText: l10n.fieldServerHost,
+              hintText: 'host[:port]',
+              prefixIcon: const Icon(Icons.dns_outlined),
               border: const OutlineInputBorder(),
             ),
           ),