feat(electrobun): wire PTY daemon into terminal tabs via Electrobun RPC

- Bun process connects to agor-ptyd via PtyClient (5 retries, exponential backoff)
- RPC bridge: 5 request handlers (create/write/resize/unsubscribe/close)
- Daemon output forwarded to WebView as pty.output messages (base64 passthrough)
- Terminal.svelte: real PTY sessions via RPC instead of echo mode
- Shared RPC schema at src/shared/pty-rpc-schema.ts
- Fixed pty-client.ts protocol: base64 string for data (was number array)
- TerminalTabs passes sessionId to Terminal component

ui-electrobun/src/mainview/TerminalTabs.svelte

@@ -69,7 +69,7 @@
 <!-- Wrapper: uses flex to push tab bar to bottom when terminal is collapsed -->
 <div class="term-wrapper" style="--accent: {accent}">
   <!-- Tab bar: always visible, acts as divider -->
-  <div class="term-bar" onmousedown={blurTerminal}>
+  <div class="term-bar" role="toolbar" aria-label="Terminal tabs" tabindex="-1" onmousedown={blurTerminal}>
     <button
       class="expand-btn"
       onclick={toggleExpand}
@@ -91,18 +91,25 @@
 
     <div class="term-tabs" role="tablist">
       {#each tabs as tab (tab.id)}
-        <button
+        <!-- div+role="tab" allows a nested <button> for the close action -->
+        <div
           class="term-tab"
           class:active={activeTabId === tab.id}
           role="tab"
+          tabindex={activeTabId === tab.id ? 0 : -1}
           aria-selected={activeTabId === tab.id}
           onclick={() => activateTab(tab.id)}
+          onkeydown={(e) => { if (e.key === 'Enter' || e.key === ' ') activateTab(tab.id); }}
         >
           <span class="tab-label">{tab.title}</span>
           {#if tabs.length > 1}
-            <span class="tab-close" onclick={(e) => closeTab(tab.id, e)}>×</span>
+            <button
+              class="tab-close"
+              aria-label="Close {tab.title}"
+              onclick={(e) => closeTab(tab.id, e)}
+            >×</button>
           {/if}
-        </button>
+        </div>
       {/each}
       <button class="tab-add" onclick={() => addTab()}>+</button>
     </div>
@@ -114,7 +121,7 @@
       {#each tabs as tab (tab.id)}
         {#if mounted.has(tab.id)}
           <div class="term-pane" style:display={activeTabId === tab.id ? 'flex' : 'none'}>
-            <Terminal />
+            <Terminal sessionId={tab.id} />
           </div>
         {/if}
       {/each}
@@ -198,6 +205,12 @@
     display: flex;
     align-items: center;
     justify-content: center;
+    /* Reset <button> defaults */
+    background: transparent;
+    border: none;
+    padding: 0;
+    font-family: inherit;
+    line-height: 1;
   }
   .tab-close:hover { background: var(--ctp-surface1); color: var(--ctp-red); }