feat(dictation): streaming STT via silence cut (Silero VAD)

Add a lightweight "streaming" dictation mode as a simpler alternative to the
realtime-websocket path: detect speech with Silero VAD (@ricky0123/vad-web),
cut each segment on a pause and POST it to the existing /ai-chat/transcribe
endpoint, so text appears progressively. No server changes.

- new useStreamingDictation hook (same API as useDictation), lazy-loads VAD,
  in-order seq emission, session-epoch guard against stop->start races
- new encodeWavPcm16 util (Float32 -> mono PCM16 WAV, accepted by the server)
- MicButton gains a `streaming` prop; enabled in the editor toolbar and chat
- VAD tuning: redemptionMs 640 / preSpeechPadMs 320 / minSpeechMs 96
- batch dictation kept as the fallback (streaming=false)
- deps: @ricky0123/vad-web@0.0.30, onnxruntime-web@1.27.0

Note: VAD assets load from the library CDN by default; for self-hosted/offline
set VAD_BASE_ASSET_PATH/VAD_ONNX_WASM_BASE_PATH and copy assets to public/vad/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pnpm-lock.yaml

@@ -299,6 +299,9 @@ importers:
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         specifier: 8.3.18
         version: 8.3.18(@mantine/core@8.3.18(@mantine/hooks@8.3.18(react@18.3.1))(@types/react@18.3.12)(react-dom@18.3.1(react@18.3.1))(react@18.3.1))(@mantine/hooks@8.3.18(react@18.3.1))(react-dom@18.3.1(react@18.3.1))(react@18.3.1)
+      '@ricky0123/vad-web':
+        specifier: ^0.0.30
+        version: 0.0.30
       '@slidoapp/emoji-mart':
         specifier: 5.8.7
         version: 5.8.7
@@ -374,6 +377,9 @@ importers:
       mitt:
         specifier: 3.0.1
         version: 3.0.1
+      onnxruntime-web:
+        specifier: ^1.27.0
+        version: 1.27.0
       posthog-js:
         specifier: 1.372.2
         version: 1.372.2
@@ -4205,6 +4211,9 @@ packages:
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     resolution: {integrity: sha512-42aWfPrimMfDKDi4YegyS7x+/0tlzaqwPQCULLanv3DMIlu96KTJR0fM5isWX2UViOqlGnX6YFgqWepcX+XMNg==}
 
+  '@ricky0123/vad-web@0.0.30':
+    resolution: {integrity: sha512-cJyYrh4YeeUBJcbR9Bic/bFDyB9qBkAepvpuWM3vLxnAi7bC3VHzf51UeNdT+OtY4D7MLAgV8iJMc4z41ZnaWg==}
+
   '@rolldown/binding-android-arm64@1.0.0-rc.12':
     resolution: {integrity: sha512-pv1y2Fv0JybcykuiiD3qBOBdz6RteYojRFY1d+b95WVuzx211CRh+ytI/+9iVyWQ6koTh5dawe4S/yRfOFjgaA==}
     engines: {node: ^20.19.0 || >=22.12.0}
@@ -5253,6 +5262,7 @@ packages:
 
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+    deprecated: Potential CWE-502 - Update to 1.3.1 or higher
 
   '@unrs/resolver-binding-android-arm-eabi@1.11.1':
     resolution: {integrity: sha512-ppLRUgHVaGRWUx0R0Ut06Mjo9gBaBkg3v/8AxusGLhsIotbBLuRk51rAzqLC8gq6NyyAojEXglNjzf6R948DNw==}
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     hasBin: true
 
+  flatbuffers@25.9.23:
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+
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@@ -7188,6 +7201,9 @@ packages:
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+
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     engines: {node: '>=6'}
 
+  onnxruntime-common@1.27.0:
+    resolution: {integrity: sha512-3KxL5wIVqa8Ex08jxSzncm9CMgw8CjOFyOQ7SxvG9o0cVLlhTNKXyIQuTbtX4tGPJEf73OER2xrjt4HJSBL4ow==}
+
+  onnxruntime-web@1.27.0:
+    resolution: {integrity: sha512-ogDLsqIozHZwifPuN37OproAo0byX6t43/bP8GzeZWBWD6MOGExswFAx3up4NS/vvWBOg2u2PXomDt3rMmdQSg==}
+
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     engines: {node: '>=12'}
@@ -8912,6 +8934,9 @@ packages:
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+  platform@1.3.6:
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+
   pluralize@8.0.0:
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     engines: {node: '>=4'}
@@ -9645,6 +9670,7 @@ packages:
 
   sliced@1.0.1:
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+    deprecated: Unsupported
 
   socket.io-adapter@2.5.4:
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@@ -14568,6 +14594,10 @@ snapshots:
 
   '@remirror/core-constants@3.0.0': {}
 
+  '@ricky0123/vad-web@0.0.30':
+    dependencies:
+      onnxruntime-web: 1.27.0
+
   '@rolldown/binding-android-arm64@1.0.0-rc.12':
     optional: true
 
@@ -17812,6 +17842,8 @@ snapshots:
 
   flat@5.0.2: {}
 
+  flatbuffers@25.9.23: {}
+
   flatted@3.4.2: {}
 
   follow-redirects@1.16.0: {}
@@ -17970,6 +18002,8 @@ snapshots:
 
   graceful-fs@4.2.11: {}
 
+  guid-typescript@1.0.9: {}
+
   hachure-fill@0.5.2: {}
 
   handlebars@4.7.9:
@@ -19587,6 +19621,17 @@ snapshots:
     dependencies:
       mimic-fn: 2.1.0
 
+  onnxruntime-common@1.27.0: {}
+
+  onnxruntime-web@1.27.0:
+    dependencies:
+      flatbuffers: 25.9.23
+      guid-typescript: 1.0.9
+      long: 5.3.2
+      onnxruntime-common: 1.27.0
+      platform: 1.3.6
+      protobufjs: 7.5.8
+
   open@8.4.2:
     dependencies:
       define-lazy-prop: 2.0.0
@@ -19911,6 +19956,8 @@ snapshots:
       mlly: 1.8.0
       pathe: 2.0.3
 
+  platform@1.3.6: {}
+
   pluralize@8.0.0: {}
 
   png-chunk-text@1.0.0: {}