Rename heatmap label to Contribution Calendar

[Arpan200502]

Closes #6849

This PR renames the user-facing "Heatmap" feature in the profile section to "Contribution Calendar" to better reflect its purpose as a temporal activity overview (similar to GitHub’s contribution graph), rather than a geographic heatmap.

Changes

• Updated user-facing labels from "Heatmap" → "Contribution Calendar"
• Updated relevant i18n keys in config/locales/en.yml
• Renamed related translation keys (e.g., edit_heatmap → edit_contribution_calendar, show_heatmap_in_public → show_contribution_calendar_in_public)
• Updated success messages and UI text accordingly
• Updated controller tests to reflect new text
• Preserved the existing heatmaps namespace to maintain correct Rails i18n scoping and avoid breaking relative lookups

Notes

• Based on reviewer feedback, translation keys were updated while keeping namespace structure intact
• Changes are limited to en.yml as other locales are managed externally via TranslateWiki

Previous PR

This replaces #6884.

The previous PR was unintentionally closed after a rebase operation temporarily aligned the branch with master, resulting in no diff. Since GitHub treats such branches as having no changes, the PR was automatically closed and could not be reopened once the history diverged again.

This PR restores the intended changes with a clean, linear commit history.

[pablobm]

I think this is almost there. There's just the detail of the capitalisation. I think there has been both a misunderstanding and an ambiguity. Let's see if we can clarify all this.

1. At Rename heatmap label to Contribution Calendar #6932 (comment), I think there's a misunderstanding by @Arpan200502. The comment is about how "heatmap" was replaced with "Contribution Calendar". In that specific instance, there's an inconsistency: it goes from non-capitalised to capitalised.
2. At Rename heatmap label to Contribution Calendar #6932 (comment) however, there's an ambiguity. "Heatmap" is replaced with "Contribution Calendar", which is not necessarily wrong. We go from a single-word noun, capitalised, to a two-word noun whose capitalisation we haven't discussed: should it be capitalised as "Contribution Calendar" or as "Contribution calendar"?

Personally, I think it should be "Contribution Calendar" in all instances. This is even for the uncapitalised "heatmap" one which I think was a prior mistake as it refers to a distinct entity in the UI that should be capitalised, as opposed to a general term that could refer to many things.

Also: @Arpan200502, when offered feedback, please don't just accept it blindly. Stop for a moment, think about what you are being asked, and reason about what to do. If you think that some things should stay as they are, they do say so and explain your rationale. Part of the job of a Software Engineer is to push back when it makes sense.

[tomhughes]

Well as I was reading it the previous behaviour was that is was capitalised when used in a title (where all main words were capitalised) or at the start of a sentence.

I was aiming to preserve that, so capitalise both words in a title, the first word at the start of a sentence, and neither in the middle of a sentence.

[pablobm]

Oh, title case. I hate title case 😆