Add processing spinner and progress indicator

[azaitsev]

Motivation

• Provide clear feedback to users while uploaded FIT/TCX files are being parsed so they know processing is in progress.

Description

• Add CSS for a spinner, progress bar, and a .hidden helper to src/static/index.html to style the processing UI.
• Insert a #processingStatus block in the files panel that contains the spinner, message (#processingMessage), and progress bar (#processingProgressBar).
• Add UI references for the new elements and implement setProcessingState(isProcessingFiles, processingMessageText) to show/hide the processing UI.
• Wire setProcessingState into the upload flow by calling it before/after the remote parse call in addSelectedFiles so the indicator is shown while fetchParsedSeries runs.

Testing

• Started the app with uvicorn src.app:app --host 0.0.0.0 --port 8000 which completed application startup successfully.
• Ran an automated Playwright script that opened the index page and saved a screenshot to artifacts/processing-status.png to verify the processing UI renders, and the script completed successfully.

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Codex Task