fix(embed): display actual model name from QMD_EMBED_MODEL env var

[JasonOA888]

Problem

When using QMD_EMBED_MODEL to switch to a different embedding model, qmd embed still displays Model: embeddinggemma even though the correct model is downloaded and used.

export QMD_EMBED_MODEL="hf:Qwen/Qwen3-Embedding-0.6B-GGUF/Qwen3-Embedding-0.6B-Q8_0.gguf"
qmd embed -f
# Output: Model: embeddinggemma  <-- incorrect

Root Cause

Two different DEFAULT_EMBED_MODEL constants exist:

• store.ts:47 - hardcoded display label "embeddinggemma"
• llm.ts:196 - actual model URI (respects QMD_EMBED_MODEL env var)

qmd.ts was importing and using the wrong constant for display.

Solution

1. Add extractModelDisplayName() helper to extract readable name from model URI
2. Use DEFAULT_EMBED_MODEL_URI (llm.ts) instead of DEFAULT_EMBED_MODEL (store.ts)
3. Apply extractModelDisplayName() when printing the model name

Result

Now correctly shows:

Model: embeddinggemma-300M-Q8_0      # for default model
Model: Qwen3-Embedding-0.6B-Q8_0     # for custom models

Fixes #328