feat(wallets): in-app folder picker + roomier, full-width list controls

Wallets dialog refinements:
- More space between wallet cards (cardGap: spacingSm -> spacingMd).
- "Scan another folder" is now always visible as a full-width button (no
  expandable toggle) that opens the new picker.
- The Create-wallet row and the scan button span the full list width, so their
  right edges align with the wallet cards / Open|Import buttons.

New in-app folder picker (folder_picker.h): a Material-styled modal replacing
the raw path input. Browse the filesystem (Up / Home / click a sub-folder),
see the *.dat wallet files present + a count, and pick a folder to scan. The
picker takes over the modal surface while open (the overlay framework doesn't
nest); it remembers the last browsed location across opens. Fixed-height card
(no auto-height feedback), UTF-8-aware truncation, full-node icons confirmed
in the embedded MaterialIcons range. Added a modal-folder-picker sweep surface;
verified at 100% + 150%, dark + light.

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

src/util/i18n.cpp

@@ -266,11 +266,8 @@ void I18n::loadBuiltinEnglish()
         strings_["wallets_never"] = "Never";
         strings_["wallets_import_hint"] = "Import to open";
         strings_["wallets_import_tt"] = "This wallet is outside the data directory. Opening it will copy it in first (coming soon).";
-        strings_["wallets_add_folder"] = "Also scan another folder for wallet files:";
-        strings_["wallets_add_folder_toggle"] = "+ Scan another folder for wallets\xE2\x80\xA6";
         strings_["wallets_external_tt"] = "Outside your data directory \xE2\x80\x94 Import to copy it in.";
-        strings_["wallets_folder_hint"] = "/path/to/folder with wallet .dat files";
-        strings_["wallets_add"] = "Add folder";
+        strings_["wallets_scan_folder"] = "Scan another folder for wallets\xE2\x80\xA6";
         strings_["wallets_folder_invalid"] = "That folder doesn't exist.";
         strings_["wallets_import"] = "Import";
         strings_["wallets_new_label"] = "Create a new wallet:";
@@ -282,6 +279,14 @@ void I18n::loadBuiltinEnglish()
         strings_["wallets_import_failed"] = "Could not import that wallet file.";
         strings_["wallets_creating"] = "Creating wallet — the node will restart…";
         strings_["wallets_reveal"] = "Reveal folder";
+        // In-app folder picker (Scan another folder)
+        strings_["picker_title"] = "Select a folder to scan";
+        strings_["picker_up"] = "Up one level";
+        strings_["picker_home"] = "Home folder";
+        strings_["picker_empty"] = "This folder has no sub-folders or wallet files.";
+        strings_["picker_dat_count"] = "%d wallet file(s) in this folder";
+        strings_["picker_dat_none"] = "No wallet files directly in this folder";
+        strings_["picker_select"] = "Scan this folder";
         strings_["tt_seed_migrate"] = "Create a new seed-phrase wallet and move your funds into it";
         // Migrate-to-seed modal
         strings_["mig_title"] = "Migrate to a seed wallet";