webui: auto-refresh /props on inference start to resync model metadata

[ServeurpersoCom]

• Add no-cache headers to /props and /slots
• Throttle slot checks to 30s
• Prevent concurrent fetches with promise guard
• Trigger refresh from chat streaming for legacy and ModelSelector
• Show dynamic serverWarning when using cached data

Close #16771

Testing video, Raspberry Pi 5 + master branch + this PR :

Legacy.mp4

Cmdline used on legacy (Raspberry Pi 5) :

./build/bin/llama-server \
 -m /root/ia/models/mradermacher/OLMoE-1B-7B-0125-Instruct-i1-GGUF/OLMoE-1B-7B-0125-Instruct.i1-Q6_K.gguf \
 -ctk q8_0 -ctv q8_0 -fa on \
 --jinja --ctx-size 8192 --mlock --port 8081

./build/bin/llama-server \
 -m /root/ia/models/mradermacher/Qwen3-30B-A3B-Instruct-2507-i1-GGUF/Qwen3-30B-A3B-Instruct-2507.i1-Q4_K_M.gguf \
 --temp 0.7 --top-p 0.8 --top-k 20 --min-p 0 \
 -ctk q8_0 -ctv q8_0 -fa on \
 --jinja --ctx-size 4096 --port 8081

./build/bin/llama-server \
 -m /root/ia/models/lmstudio-community/gpt-oss-20b-GGUF/gpt-oss-20b-MXFP4.gguf \
 -ctk q8_0 -ctv q8_0 -fa on \
 --jinja --ctx-size 4096 --port 8081

ModelSelector enabled (same UI refresh behavior on first chunk):

ModelSelector.mp4