improve upload and inspect page UX

[Alan0893]

The vector database dropdown defaulted to "Create New" instead of the first existing database, document deletion was implemented in the backend code but never exposed in the UI, empty vector stores showed no feedback, and the table CSS used hardcoded dark-theme colors that looked off in light mode.

The Inspect sub-pages (Models, Providers, Vector Databases, Shields) had no error handling: if the LlamaStack backend was unreachable, the pages would crash with a raw Python traceback. Additionally, the Models page called models.list() twice per render, and the Providers table was hardcoded to 500px width.

Upload page changes:

• Default selection now picks the first existing vector database instead of "Create New" when databases exist
• Added a delete button (trash icon) on each document row, wired to the existing _delete_file_from_vector_store API
• Added an empty state message ("No documents found...") when a vector store has no documents
• Table borders now use rgba() values that adapt to both light and dark Streamlit themes
• File listing errors now display as st.warning instead of print() to console

Inspect page changes:

• All four sub-pages (Models, Providers, Vector Databases, Shields) now wrap API calls in try/except with user-friendly error messages
• Added st.spinner loading indicators while data is being fetched
• Providers table uses use_container_width=True instead of hardcoded width=500 for responsive layout
• Models page no longer calls models.list() twice per render
• Fixed variable shadowing in Providers where loop variable providers shadowed the function name

Tests

Suite	Passed	Skipped	Failed
Unit tests	23	0	0
Integration tests	24	1	0
LlamaStack integration	4	0	0
UI E2E (Playwright)	24	2	0
Total	75	3	0

Skipped tests:

• test_chat_page_loads — requires full Streamlit app context
• test_maas_chat_completion_direct_mode — requires live MaaS model endpoint
• test_maas_model_selection — requires live MaaS model endpoint

[gurvinder]

Separate out code into commits, each "feature" or isolated functionality change should be its own commit so it can be independently rolled back without touching any other part of the code.

Example Commits:

• Smart default — pick the first real DB, not "Create New"
• border-top rgba(128, 128, 128, 0.3) and background-color rgba(128, 128, 128, 0.06) change
• etc...