fix(steam): restore achievement loading for real-world games

[MotherSphere]

Summary

Three independent bugs in src-tauri/src/steam.rs that combine to make Samira return empty achievement lists for most non-trivial games. All happen to be hidden by happy-path luck on small/simple games, which is why upstream testing didn't catch them.

I hit these while building a downstream fork (Project-Colony/SAM-Colony-Edition) — sending the bug-fix portion upstream because it benefits everyone using Samira on AAA titles.

The bugs

1. CallbackHandle leak in start_client

client.register_callback(move |_data: UserStatsReceived| { ... });
//                                                                ^ return value discarded

register_callback returns a CallbackHandle whose Drop impl synchronously unregisters the callback (steamworks 0.12.2/src/callback.rs#L143). Discarding the return value drops the handle right away, removing the callback before Steam can ever fire it. The 1-second wait loop then always times out, and load_achievements proceeds against unready stats.

Fix: bind the handle to a named local for the duration of the wait loop (let _stats_cb = ...). Also reorder request_user_stats() to fire after the callback is registered (otherwise a fast pipe can race and we miss the event even with the handle held).

2. break; after first ACHIEVEMENTS block in load_achievement_icons

Steam represents achievements as 32-bit bitfields. Games with more than 32 achievements (Elden Ring: 42, Skyrim: 75, RDR2: 50+, etc.) have multiple ACHIEVEMENTS entries in the schema. The current loop reads the first block, then break;s, silently dropping every icon past index 31.

Fix: remove the break;, iterate all ACHIEVEMENTS blocks.

3. .unwrap() panics in load_achievements

Three call sites unwrap Result<&str, ()> from get_achievement_display_attribute and Result<bool, ()> from get(). Either fails for any achievement whose status hasn't loaded yet (which, before fix #1, was every achievement on every game). The panic is caught by catch_unwind and converted to Err, which main.rs maps to Vec::new(). So a single transient hiccup gives the UI "0/0" with no error surfacing.

Also: user_stats.get_achievement_names() panics inside steamworks-rs when the game has zero achievements (it calls .expect() on get_num_achievements). This crashes Samira when the user picks a no-achievement game.

Fix: replace .unwrap() with .unwrap_or_default() / .unwrap_or(false), and short-circuit load_achievements when get_num_achievements() returns 0.

Test plan

• Builds clean on Linux (cargo check + full tauri build)
• Elden Ring (AppID 1245620, 42 achievements): before = 0/42 with no error, with only fix 1 = 21/42 (half missing icons), with all three = 42/42 ✅
• Counter-Strike 2 (AppID 730, 1 achievement): unchanged ✅
• Game with no achievements: before = SIGABRT, after = renders "0/0" cleanly ✅
• panic::catch_unwind boundary preserved, so any unexpected residual panic still surfaces as Err(String) instead of crashing the process