chore(hotkeys): remove sloppak-convert library hotkey (#594)

* chore(hotkeys): remove sloppak-convert library hotkey

Removes the 'c' keyboard shortcut for converting library entries to
.sloppak. The shortcut was defined in two places:

- The no-op registerShortcut() entry that only existed to show in the
  ? help panel (the Sloppak Converter plugin handles conversion and
  can register its own shortcut via window.registerShortcut).
- The c dispatch in the library-entry keydown handler
  ({ c: 'button.sloppak-convert-btn', ... }) that triggered the
  plugin button.

* test+docs: update tests & CHANGELOG for removed `c` convert hotkey

The previous commit removed the `c` library hotkey but left three
assertions in tests/browser/keyboard-shortcuts.spec.ts that require it,
which fail deterministically (the two registry tests read window._panels
directly, independent of environment):
- should list all registered shortcuts (required {key:'c',scope:'library'})
- should have correct shortcut scopes (expected library::c)
- should show library shortcuts in help modal (Convert library entry / c)

Drop those assertions and record the removal under CHANGELOG
[Unreleased] -> Removed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: byrongamatos <xasiklas@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

static/app.js

@@ -923,15 +923,13 @@ document.addEventListener('keydown', (e) => {
     // library entry — works on both grid cards and tree rows. Each
     // dispatches to a button class that the entry markup already
     // exposes, so plugins can keep owning the actual behavior:
-    //   c → .sloppak-convert-btn  (Sloppak Converter plugin)
     //   f → .fav-btn              (favorite heart toggle)
     //   e → .edit-btn             (edit metadata modal)
     // No-op when no entry is currently focused / selected, when the
-    // entry doesn't expose the requested button (e.g. a sloppak
-    // entry has no convert button), or when the button is disabled.
+    // entry doesn't expose the requested button, or when the button is disabled.
     // Bails on text input / drawer focus so single-letter typing in
     // inputs still works.
-    const entryShortcut = { c: 'button.sloppak-convert-btn', f: 'button.fav-btn', e: 'button.edit-btn' }[e.key.toLowerCase()];
+    const entryShortcut = { f: 'button.fav-btn', e: 'button.edit-btn' }[e.key.toLowerCase()];
     if (entryShortcut) {
         if (_isInsideInteractiveControl(document.activeElement)) return;
         const ae = document.activeElement;
@@ -9435,15 +9433,6 @@ registerShortcut({
     }
 });
 
-registerShortcut({
-    key: 'c',
-    description: 'Convert library entry to .sloppak',
-    scope: 'library',
-    handler: () => {
-        // Handled by library navigation - this is for documentation only
-    }
-});
-
 registerShortcut({
     key: 'f',
     description: 'Toggle favorite',