feat: add share button for challenge links

[kp-krish]

Checklist:

• I have read and followed the contribution guidelines.
• I have read and followed the how to open a pull request guide.
• My pull request targets the main branch of the repo.
• I have tested these changes locally on my machine.

Closes #1295

Adds a share button to the challenge bottom bar so campers can share a link to the exact step they are on, rather than posting screenshots of the instructions or their code when asking for help on Discord or the forum.

Tapping it opens the native share sheet with, for example:

Build a Curriculum Outline - Step 1

https://www.freecodecamp.org/learn/responsive-web-design-v9/workshop-curriculum-outline/step-1

The share sheet includes "Copy to clipboard", so this also covers the clipboard behaviour described in the issue, while letting campers send the link straight to Discord in one step.

Implementation notes

• No new dependency. share_plus is already in pubspec.yaml, and this follows the same SharePlus.instance.share(ShareParams(...)) pattern already used by the news tutorial view.
• The button is gray, per the review feedback on the earlier attempt at this issue. It reuses the existing _panelIconButton helper, so it picks up the same styling as the instruction/preview/console buttons beside it automatically.
• The URL is no longer duplicated. genForumLink already built the same /learn/<superBlock>/<block>/<dashedName> URL inline. That is now extracted into a challengeUrl helper in challenge_utils.dart and reused in both places.
• The block name is included alongside the challenge title, matching the naming genForumLink already uses. Sharing just "Step 1" would be ambiguous, since many workshops have a step with that title.
• The tooltip is localized through a new share_challenge key rather than hardcoded, in keeping with fix: move hardcoded strings to translations #1794. Only app_en.arb is edited by hand; app_es.arb and app_pt.arb are left to Crowdin.

Scope

The button is added to ChallengeView, which covers the code-editor challenges and daily challenges. The alternative templates (python, english, multiple choice, quiz, review) each render their own bottom bar, so they are not affected. Happy to extend it to those in this PR if you would prefer.

Testing

Verified on Flutter 3.44.9 (the version pinned in CI):

• flutter analyze --no-fatal-warnings reports no new issues
• flutter test unit (29 tests) and flutter test services (37 tests) pass, including a new unit test covering challengeUrl
• Ran the debug build on an Android 36 emulator and confirmed the button renders in the bottom bar with the same styling as its neighbours, and that the share sheet opens with the correct title and step URL

I have screenshots of the button and the share sheet and am happy to add them here.