feat: add OPDS search support & next/prev page navigation (#1462)

## Summary

**What is the goal of this PR?**
Adds OPDS search support, allowing users to search a catalog directly
from the book browser when the server exposes an OpenSearch template.

**What changes are included?**
- `OpdsParser`: parses the OpenSearch template URL from feed-level
`<link rel="search">` elements and exposes it via `getSearchTemplate()`
- `OpdsBookBrowserActivity`: fetches and stores the search template
after each feed load; shows a Search hint on the Left button when a
template is available; launches the existing `KeyboardEntryActivity` for
query input; URL-encodes the query and fetches the result feed
- Absolute search result URLs are handled correctly in `fetchFeed`
(skips prepending the server base URL)
- A `consumeConfirm` guard prevents the Confirm release that submits the
keyboard from immediately triggering a book download on the first
browsing frame after search results load

## Additional Context

- Search is silently unavailable if the server does not advertise an
OpenSearch template — no UI change in that case
- Tested against a Calibre-Web OPDS endpoint which exposes `<link
rel="search" type="application/opensearchdescription+xml">`
- The inline URL encoder in `performSearch` was necessary as
`StringUtils` has no such utility; worth considering extracting to
`StringUtils` in a follow-up
- No new dependencies introduced

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**NO**_

---------

Co-authored-by: kira <rammah@tuta.io>
Co-authored-by: Justin Mitchell <justin@jmitch.com>

src/activities/browser/OpdsBookBrowserActivity.h

@@ -11,21 +11,13 @@
 /**
  * Activity for browsing and downloading books from an OPDS server.
  * Supports navigation through catalog hierarchy and downloading EPUBs.
- * When WiFi connection fails, launches WiFi selection to let user connect.
  */
 class OpdsBookBrowserActivity final : public Activity {
  public:
-  enum class BrowserState {
-    CHECK_WIFI,      // Checking WiFi connection
-    WIFI_SELECTION,  // WiFi selection subactivity is active
-    LOADING,         // Fetching OPDS feed
-    BROWSING,        // Displaying entries (navigation or books)
-    DOWNLOADING,     // Downloading selected EPUB
-    ERROR            // Error state with message
-  };
+  enum class BrowserState { CHECK_WIFI, WIFI_SELECTION, LOADING, BROWSING, DOWNLOADING, ERROR, SEARCH_INPUT };
 
   explicit OpdsBookBrowserActivity(GfxRenderer& renderer, MappedInputManager& mappedInput)
-      : Activity("OpdsBookBrowser", renderer, mappedInput) {}
+      : Activity("OpdsBookBrowser", renderer, mappedInput), buttonNavigator() {}
 
   void onEnter() override;
   void onExit() override;
@@ -36,8 +28,11 @@ class OpdsBookBrowserActivity final : public Activity {
   ButtonNavigator buttonNavigator;
   BrowserState state = BrowserState::LOADING;
   std::vector<OpdsEntry> entries;
-  std::vector<std::string> navigationHistory;  // Stack of previous feed paths for back navigation
-  std::string currentPath;                     // Current feed path being displayed
+  std::vector<std::string> navigationHistory;
+  std::string currentPath;
+  std::string searchTemplate;
+  bool consumeConfirm = false;
+  bool consumeBack = false;  // Added missing member
   int selectorIndex = 0;
   std::string errorMessage;
   std::string statusMessage;
@@ -51,5 +46,7 @@ class OpdsBookBrowserActivity final : public Activity {
   void navigateToEntry(const OpdsEntry& entry);
   void navigateBack();
   void downloadBook(const OpdsEntry& book);
+  void launchSearch();
+  void performSearch(const std::string& query);
   bool preventAutoSleep() override { return true; }
 };