fix: replace deprecated use_container_width with width="stretch" (closes #118)

[voidborne-d]

Closes #118.

Summary

Streamlit prints a DeprecationWarning for every use_container_width call and will remove the parameter after 2025-12-31:

Please replace `use_container_width` with `width`.
`use_container_width` will be removed after 2025-12-31.
For `use_container_width=True`, use `width='stretch'`.
For `use_container_width=False`, use `width='content'`.

This PR:

• Replaces all 47 use_container_width=True call sites with width="stretch" across web/pipelines/, web/components/, web/pages/. There were no =False call sites in the tree, so the mapping is purely True -> "stretch".
• Bumps streamlit>=1.40.0 -> streamlit>=1.50.0 in pyproject.toml. The uv.lock already pins 1.53.1, so this just prevents an unsupported install on a fresh user environment that resolves to an older Streamlit.

Why bump the floor to 1.50?

I verified empirically across released Streamlit versions when the width keyword landed for the widgets actually used here:

widget	width= introduced
st.image	1.46
st.button	1.48
st.download_button	1.48

Anything below 1.48 will TypeError on st.button(..., width="stretch"). 1.50 is a round, post-rollout floor that comfortably covers all three (and is older than this project — Pixelle-Video was created 2025-11, Streamlit 1.50 shipped Aug 2025).

Files changed

• pyproject.toml (+1/-1, dep floor)
• web/components/{content_input,digital_tts_config,output_preview,settings,style_config}.py
• web/pages/2_📚_History.py
• web/pipelines/{action_transfer,asset_based,digital_human,i2v}.py

Verified locally

• ast.parse clean across all 10 modified .py files (including the emoji-named 2_📚_History.py).
• inspect.signature(st.button)["width"] / st.download_button / st.image return non-empty Width annotation with default "content" on Streamlit 1.50.0 — confirms width="stretch" is a documented call shape, not just an undocumented kwarg.
• Repo has no tests/ directory and no PR-gating CI workflow (only docs.yml), so no test surface to update.

Out of scope (intentionally)

• Issue asset_default.html does not follow template naming convention #119 (asset_default.html naming convention) — separate concern, separate PR if desired.
• uv.lock regeneration — left untouched. Pinned 1.53.1 already satisfies >=1.50.0. A uv lock rerun on a different Python version produces ~3,900 lines of platform-metadata churn unrelated to this fix; the maintainer's next routine sync will pick it up cleanly.

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