feat: 5204 - multi-product price addition

[monsieurtanuki]

What

• Now we can add several product prices in one shot, and even offline.
• For the time being, it's just that on all the "add a single price" pages we can add and remove products (and their prices).
• The products are found by barcodes, locally, and from the server if needed.
• Many possible improvements in next PRs:
  • more obvious "Add prices" / "Add receipt" buttons - but I don't know where to put them in the app
  • more refined product selection - scanner, list of previous "price" products, list of local products, full text search, ...

Screenshots

top	middle	bottom

Et voilà:

Fixes bug(s)

• Fixes: Replicate the 'Add Receipts' view from the Open Prices web app #5204

Impacted files

• app_en.arb: added 3 labels and removed one redundant one
• app_fr.arb: added 3 labels and removed one redundant one
• background_task_add_price.dart: now we manage multiple products, in Lists
• price_amount_card.dart: small UI/UX differences for a single product among several products; minor code simplification
• price_model.dart: now we manage multiple products
• price_product_search_page.dart: added "no initial product" case
• product_price_add_page.dart: now we manage multiple products

[teolemon]

Woohoo 🎉

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0.96154% with 103 lines in your changes missing coverage. Please review.

Project coverage is 7.38%. Comparing base (4d9c7fc) to head (f9a3e61).
Report is 189 commits behind head on develop.

❗ Current head f9a3e61 differs from pull request most recent head 19f867c

Please upload reports for the commit 19f867c to get more accurate results.

Files	Patch %	Lines
..._app/lib/background/background_task_add_price.dart	0.00%	43 Missing ⚠️
...kages/smooth_app/lib/pages/prices/price_model.dart	0.00%	21 Missing ⚠️
...smooth_app/lib/pages/prices/price_amount_card.dart	0.00%	20 Missing ⚠️
...h_app/lib/pages/prices/product_price_add_page.dart	0.00%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #5375      +/-   ##
==========================================
- Coverage     9.54%   7.38%   -2.17%     
==========================================
  Files          325     384      +59     
  Lines        16411   19633    +3222     
==========================================
- Hits          1567    1449     -118     
- Misses       14844   18184    +3340     

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[monsieurtanuki]

Thank you @teolemon for your quick review!

Play with it and tell me what you like / don't like.

cc @raphael0202 @raphodn