[feat](trx-frontend-http): add BookmarkStore backed by pickledb

Add pickledb and dirs dependencies. Introduce BookmarkStore wrapping
PickleDb behind Arc<RwLock<>> with list/get/insert/upsert/remove ops.
Keys stored as "bm:{id}" in ~/.config/trx-rs/bookmarks.db; falls back
to ./bookmarks.db when config dir is unavailable.

Wire BookmarkStore into build_server() as actix-web app_data so all
request handlers can share the store.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

Cargo.lock

@@ -1586,6 +1586,16 @@ version = "2.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9b4f627cb1b25917193a259e49bdad08f671f8d9708acfd5fe0a8c1455d87220"
 
+[[package]]
+name = "pickledb"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c53a5ade47760e8cc4986bdc5e72daeffaaaee64cbc374f9cfe0a00c1cd87b1f"
+dependencies = [
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "pin-project-lite"
 version = "0.2.16"
@@ -2503,8 +2513,10 @@ dependencies = [
  "actix-web",
  "actix-ws",
  "bytes",
+ "dirs",
  "futures-util",
  "hex",
+ "pickledb",
  "rand 0.8.5",
  "serde",
  "serde_json",