feat(FileUploader): empty the uploader after a successful submit

[thorwhalen]

Fixes i2mint/front#24 (needs the clear_after_submit field from i2mint/front#35, merged; the code reads it with a default of True so it also runs on an older front).

After a successful, non-auto_submit submit, every FileUploader of the section (also inside a MultiSourceInput) with clear_after_submit (default True) is emptied, so the next batch doesn't start from the previous selection. Streamlit can only empty an uploader by giving it a new key, so the uploader keeps an st.empty() slot, bumps a key generation in session state and draws a fresh uploader into the same slot in the same run: no rerun, so the output and anything the function or on_submit draws stay in place. clear_after_submit=False keeps the old behaviour. The widget now has an explicit key (this also fixes a crash with two identical unkeyed uploaders on a page).

Tests: tests/test_file_uploader_clear.py runs the app headlessly with AppTest (clears, output kept, second batch clean, opt-out, failed submit keeps the selection, multi-source).

Adversarial review (fresh Opus subagent, two rounds): round 1 found my first design (rerun + stashed output) lost anything drawn by the function/on_submit, and keyed the stash by section name: replaced by the in-place redraw. Round 2 found a stale slot could be redrawn for a source not drawn in this run (apps cached across runs): slots are now reset at the start of each run. Deferred: uploaders inside a KwargsInput are not cleared (#14).

Dependents gate (priv test-dependents streamlitfront --transitive): pass.

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