fix(rss): bypass WAF bot blocking with browser User-Agent and fix search feed hang

[heberjeur]

What & Why

Fixes #1164
Related to #13

When subscribing to feeds protected by Cloudflare WAF / Bot Fight Mode (such as https://www.phoronix.com/rss.php), the request was blocked with HTTP 403 Forbidden and a Cloudflare HTML challenge page, throwing:
Invalid XML: Error on line 1: At line 1, column 766: not well-formed (invalid token)

Additionally:

1. OkHttp's BridgeInterceptor was injecting its default User-Agent: okhttp/..., which was not replaced when checking header("User-Agent") == null.
2. BestIconFinder was downloading up to 15 icon candidate variants sequentially in cleartext http://, causing the subscribe search dialog to hang in the "Searching..." state for 30+ seconds.
3. In AbstractRssRepository.kt, searchedFeed.title.decodeHTML()!! had a potential NPE risk.
4. In SubscribeViewModel.kt, searchFeed aborted if groupsFlow was initially empty.

Fixes Applied

• Browser User-Agent (OkHttpClientModule.kt): Set standard Android browser User-Agent (Mozilla/5.0 ...) at both application and network interceptor levels, filtering out OkHttp's internal default User-Agent.
• Fast & Safe Icon Finder (BestIconFinder.kt): Default to https://, limit icon probing to a max of 4 candidates, and wrap all icon fetching in runCatching so icon resolution never blocks or fails feed addition.
• Robust RSS Search & Sync (RssHelper.kt): Check response.commonIsSuccessful before attempting to parse bodies as XML, logging clear HTTP error codes rather than passing HTML error pages into ROME's XmlReader.
• Safe Subscribe & Null-Safety (AbstractRssRepository.kt & SubscribeViewModel.kt): Safe fallback for feed title decoding and empty group handling.
• Unit Tests: Added unit and real network integration tests in OkHttpClientModuleTest and RssHelperTest.

Tested

• Tested feed search with direct feed URL (https://www.phoronix.com/rss.php) and website URL (https://www.phoronix.com).
• Ran unit tests: ./gradlew testGithubReleaseUnitTest -> All passed.
• Built release APK: ./gradlew assembleGithubRelease -> BUILD SUCCESSFUL.