feat: Modernize OPF mobile app page with environmental impact emphasis (#823)

Replaces the outdated Open Products Facts mobile app page with a modern
landing page that emphasizes OPF's core value proposition: environmental
impact, repairability, and circular economy. Food features are now
secondary.

## Changes

- **Content reordering**: Environmental impact (CO2), repairability
scores, and circular economy now featured first; food content moved to
later sections
- **App linking**: All store links point to Open Food Facts app (which
supports OPF product scanning)
- **Template upgrade**: Adopted modern OFF landing page structure with
responsive blocks, improved UX, and visual consistency
- **Analytics**: Updated UTM source from `off` to `opf` for proper
tracking

## Key sections (in order)
1. Environmental impact & CO2 footprint
2. Repairability scores with visual indicator
3. Circular economy & repair options
4. Food products (Nutri-Score, nutrition) - now secondary
5. Cosmetics, pet food, price tracking

File changed: `lang/opf/en/texts/open-products-facts-mobile-app.html`
(75 → 327 lines)

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Mobile page for Open Products Facts</issue_title>
> <issue_description>### Story
> - Can you replace
lang/opf/en/texts/open-products-facts-mobile-app.html by a modified for
Open Products Facts version of
lang/off/en/texts/open-products-facts-mobile-app.html, taking into
account that we point people to the Open Food Facts app, which has
features for Open Products Facts. The emphasis should be on the benefits
of Open Products Facts like carbon footprint, reparability index, repair
options (you can find them in lang/opf/en/texts/discover.html), and food
should be a secondary feature.
> 
> ### Part of
> - <!-- Insert Story issue number here -->
> 
> </issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes #822

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: teolemon <[email protected]>

Author: Copilot