Add terminal preview panel, fix layout issues, add favicon

[utsengar]

Summary

• Adds a generic TerminalWorkspace type + TerminalPreview component: a small, terminal-styled reference panel for coding-challenge patterns that shows the run/test commands a candidate should use. CodeFlow itself never knows what language or toolchain a problem uses — it just displays whatever commands the problem declares (e.g. ./run.sh, ./test.sh). This is a static, copyable reference, not a live shell.
• Fixes a handful of UI issues found while building this (see below).
• Adds a proper favicon (SVG primary, PNG/ICO fallbacks, web manifest) — the app previously referenced a favicon.ico that didn't exist.

UI fixes

• Duplicate instructions: coding-challenge patterns no longer show a redundant internal instructions overlay when the sidebar already displays the same content permanently — previously the only way past it was pressing Escape.
• Homepage grid touching viewport edges: the challenge card grid had no horizontal padding, so cards sat flush against the browser edge below ~1280px width. Added responsive padding; the two intro cards now also stack on narrow screens instead of squeezing side by side.
• Clipped instructions text: long paragraphs in the instructions sidebar were getting cut off mid-line instead of wrapping. Root cause: Radix ScrollArea wraps its children in an internal display: table div (to measure scroll size) that sizes to unwrapped intrinsic width instead of the space actually available. Added a targeted CSS override to force it back to a normal block box.
• Tab bar alignment: the instructions tab row had no horizontal padding, so it sat flush left, misaligned with both the content below it and the header above it.
• Inconsistent card button widths: challenge cards with different amounts of content (tags, instruction count) rendered their "Launch challenge" buttons at different widths. Cards now explicitly stretch to fill their grid cell and content areas grow to push footers to a consistent position.

Test plan

• tsc --noEmit passes clean
• npm start, confirm the homepage grid has consistent padding at various viewport widths
• Open a coding-challenge pattern, confirm no duplicate instructions overlay and the terminal preview panel renders correctly
• Confirm browser tab shows the new favicon

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