feat(B.1 #5): per-OS hotkey capability matrix with inline rejection copy

Adds src/lib/utils/hotkeyValidity.ts with validateHotkey(combo, os)
and wires it into HotkeyRecorder.svelte. Rules:

  - X11/Wayland/Linux: reject single-key combos unless the trigger
    is F13..F24 (the conventional global-shortcut escape hatch).
  - Windows: reject combos whose only modifier is a right-hand
    variant (RCtrl/RAlt) — matches Handy #966's failure mode where
    RegisterHotKey silently ignores them.
  - macOS: reject Fn-only combos and bare-key combos (non-function
    keys).
  - unknown OS: pass through — better to ship a flawed combo than
    reject one we can't validate.

When validation fails, the recorder leaves the previous known-good
hotkey intact and surfaces an inline warning-coloured sentence
explaining why, with actionable copy ("add a modifier", "use the
left-hand equivalent", etc.). The save button disappears because
settings.globalHotkey is never written to — no state drift.

Matches Handy #917 / #1019 / #966 / #956, brief item #5.

Co-authored-by: jars <jakejars@users.noreply.github.com>

src/lib/components/HotkeyRecorder.svelte

@@ -1,8 +1,21 @@
 <script lang="ts">
   import { settings, saveSettings } from "$lib/stores/page.svelte.js";
+  import { osInfo } from "$lib/utils/osInfo";
+  import { validateHotkey, type HotkeyOs } from "$lib/utils/hotkeyValidity";
 
   let recording = $state(false);
   let captured = $state(false);
+  let rejectionReason = $state<string | null>(null);
+
+  function currentOs(): HotkeyOs {
+    const info = osInfo();
+    if (!info) return "unknown";
+    const name = (info.os || "").toLowerCase();
+    if (name.startsWith("mac")) return "macos";
+    if (name.startsWith("win")) return "windows";
+    if (name.startsWith("linux")) return "linux";
+    return "unknown";
+  }
 
   // Linux webkit2gtk fires e.key === "Super" for the Windows/Super key;
   // Firefox/older Chrome may fire "OS" or "Hyper". All of these should be
@@ -109,7 +122,20 @@
     }
 
     parts.push(resolveTriggerKey(e));
-    settings.globalHotkey = parts.join("+");
+    const candidate = parts.join("+");
+
+    // Per-OS capability matrix — reject combos the backend can't
+    // reliably bind before they land in settings.globalHotkey (brief
+    // item #5). Leaves the previous, known-good hotkey intact.
+    const check = validateHotkey(candidate, currentOs());
+    if (!check.valid) {
+      rejectionReason = check.reason;
+      recording = false;
+      return;
+    }
+
+    rejectionReason = null;
+    settings.globalHotkey = candidate;
     saveSettings();
     recording = false;
     captured = true;
@@ -138,6 +164,7 @@
   function startRecording() {
     recording = true;
     captured = false;
+    rejectionReason = null;
   }
 
   let chips = $derived(settings.globalHotkey.split("+"));
@@ -155,29 +182,36 @@
   );
 </script>
 
-<button
-  type="button"
-  class="inline-flex items-center gap-1.5 rounded-xl px-4 py-2.5
-    {recording
-      ? 'bg-bg-input border-accent shadow-[0_0_0_3px_rgba(232,168,124,0.1)]'
-      : captured
-        ? 'bg-bg-input border-border animate-pulse-warm'
-        : 'bg-bg-input border-border hover:border-border'}
-    border transition-all"
-  onclick={startRecording}
-  aria-label={ariaLabel}
-  aria-pressed={recording}
->
-  <span class="sr-only" aria-live="polite">{srStatus}</span>
-  {#if recording}
-    <span class="text-[11px] text-text-tertiary italic">Hold Ctrl/Alt/Super + key...</span>
-  {:else}
-    {#each chips as chip, i}
-      {#if i > 0}
-        <span class="text-[10px] text-text-tertiary" aria-hidden="true">+</span>
-      {/if}
-      <span class="px-2 py-0.5 rounded-md bg-bg-elevated border border-border text-[11px] font-medium text-text
-        shadow-[inset_0_1px_2px_rgba(0,0,0,0.3)]">{chip}</span>
-    {/each}
+<div class="inline-flex flex-col items-start gap-1.5">
+  <button
+    type="button"
+    class="inline-flex items-center gap-1.5 rounded-xl px-4 py-2.5
+      {recording
+        ? 'bg-bg-input border-accent shadow-[0_0_0_3px_rgba(232,168,124,0.1)]'
+        : captured
+          ? 'bg-bg-input border-border animate-pulse-warm'
+          : 'bg-bg-input border-border hover:border-border'}
+      border transition-all"
+    onclick={startRecording}
+    aria-label={ariaLabel}
+    aria-pressed={recording}
+  >
+    <span class="sr-only" aria-live="polite">{srStatus}</span>
+    {#if recording}
+      <span class="text-[11px] text-text-tertiary italic">Hold Ctrl/Alt/Super + key...</span>
+    {:else}
+      {#each chips as chip, i}
+        {#if i > 0}
+          <span class="text-[10px] text-text-tertiary" aria-hidden="true">+</span>
+        {/if}
+        <span class="px-2 py-0.5 rounded-md bg-bg-elevated border border-border text-[11px] font-medium text-text
+          shadow-[inset_0_1px_2px_rgba(0,0,0,0.3)]">{chip}</span>
+      {/each}
+    {/if}
+  </button>
+  {#if rejectionReason}
+    <span class="text-[11px] text-warning" role="alert">
+      Couldn't bind that combo — {rejectionReason}.
+    </span>
   {/if}
-</button>
+</div>