refactor: fix latent bugs, invert bubble→app deps, unify color path

Whole-project review pass over the entire crate. No new features.
All function definitions preserved; layout and visibility reorganised.

Bug fixes:
- bubble: ExtractIconExW HICON pair was leaked per bubble toggle.
  Extract once into a process-wide OnceLock, reuse forever (bounded).
- usage/anthropic: parse_iso8601 was stripping the UTC offset without
  applying it — negative-offset users saw countdowns up to 14h wrong.
  Now parses signed minutes and computes utc_secs = local - off*60.
  Also rejects y<1970, mo∉[1,12], d∉[1,max_day(mo,y)] up front so
  malformed API responses can't index DAYS_IN_MONTH out of bounds.
- usage: clamp utilization to [0,100] at all four Window construction
  sites so a misbehaving server can't render "121%".
- bubble: GetDC and CreateCompatibleDC results weren't checked. Guard
  both; release the screen DC on the CreateCompatibleDC failure path.

Refactor:
- Drop type TrayIconKind = ProviderId aliases (5 sites); use ProviderId
  directly everywhere. Inline the identity-function kind_to_provider.
- Delete panel::bar_color_for shim (was just argument-reorder glue).
- Replace local scale_to_dpi fns in bubble.rs and panel.rs with
  use crate::os::dpi::scale as scale_to_dpi (brings os::dpi into the
  live import graph; was unused before).
- Delete dead PCWSTR import + #[allow(dead_code)] sentinel in
  tray/badge.rs; fold the trailing `use BOOL` into the top imports.
- Inline app::primary_dpi() to crate::os::dpi::for_system().

app.rs:
- Add update_settings(|s: &mut AppState|) helper that locks state,
  runs the closure, snapshots Settings, drops the lock, then saves
  to disk. Convert four pure-mutate-then-save callsites.

Layering: bubble.rs no longer reaches upward into crate::app::.
Introduce bubble::Callbacks (fn-pointers), OnceLock<Callbacks>, and
bubble::install_callbacks(). The wnd_proc dispatches the six prior
upward calls via a private dispatch() helper. app::run installs
callbacks once at startup; the six on_bubble_* / recheck_theme fns
are demoted from pub fn to fn.

Resource-warning logs added: dispatch() warns on uninstalled
callbacks; app_icons() warns when ExtractIconExW returns nulls.

Build: cargo build --release clean; cargo clippy reports zero new
warnings (11 pre-existing, all in untouched code).

src/panel.rs

@@ -16,9 +16,9 @@ use windows::Win32::UI::HiDpi::GetDpiForWindow;
 use windows::Win32::UI::WindowsAndMessaging::*;
 
 use crate::i18n::LocaleStrings;
+use crate::os::dpi::scale as scale_to_dpi;
 use crate::os::{to_utf16_nul as wide_str, Rgb as Color};
 use crate::usage::ProviderId;
-type TrayIconKind = ProviderId;
 
 const CLASS_NAME: &str = "ClaudeCodeUsageBubblePanel";
 const PANEL_W_LOGICAL: i32 = 280;
@@ -30,7 +30,7 @@ const RIGHT_TEXT_W_LOGICAL: i32 = 96;
 const BAR_HEIGHT_LOGICAL: i32 = 14;
 
 pub struct PanelData {
-    pub model: TrayIconKind,
+    pub model: ProviderId,
     pub session_pct: f64,
     pub session_text: String,
     pub weekly_pct: f64,
@@ -86,7 +86,7 @@ pub fn is_visible() -> bool {
         .unwrap_or(false)
 }
 
-pub fn current_model() -> Option<TrayIconKind> {
+pub fn current_model() -> Option<ProviderId> {
     lock_state().as_ref().map(|p| p.data.model)
 }
 
@@ -291,8 +291,10 @@ fn paint(hwnd: HWND, hdc: HDC) {
     } else {
         Color::from_hex("#D6D6D6")
     };
-    let session_accent = bar_color_for(data.model, data.session_pct, data.is_dark);
-    let weekly_accent = bar_color_for(data.model, data.weekly_pct, data.is_dark);
+    let session_accent =
+        crate::usage_color::bar_fill_color(data.model, data.is_dark, data.session_pct);
+    let weekly_accent =
+        crate::usage_color::bar_fill_color(data.model, data.is_dark, data.weekly_pct);
 
     unsafe {
         let bg_brush = CreateSolidBrush(COLORREF(bg.into_colorref()));
@@ -498,10 +500,6 @@ fn draw_text(
     }
 }
 
-fn bar_color_for(model: ProviderId, percent: f64, is_dark: bool) -> Color {
-    crate::usage_color::bar_fill_color(model, is_dark, percent)
-}
-
 fn clone_data() -> Option<PanelData> {
     let guard = lock_state();
     let p = guard.as_ref()?;
@@ -544,7 +542,3 @@ fn place_near(anchor: RECT, panel_w: i32, panel_h: i32) -> (i32, i32) {
     }
     (x, y)
 }
-
-fn scale_to_dpi(logical: i32, dpi: u32) -> i32 {
-    ((logical as i64) * (dpi as i64) / 96) as i32
-}