Adapt UI to small screens

[benoit74]

Rationale

Fix #89 Fix #77 Fix #63

Changes

• remove the footer which is not used (at least for now)
• fix colors and responsiveness of existing screens
• switch to Roboto font (which is the default for Vue.JS, so no need to specify one)
• implement top toolbar for mobile screens
• logic around computation of current date has been moved from uptime store to main store since it needs to be shared among components (and is hence not specific to the uptime component)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (5b2597f) 0.00% compared to head (3e2958f) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #90 +/- ## =========================================== + Coverage 0 100.00% +100.00% =========================================== Files 0 37 +37 Lines 0 1090 +1090 Branches 0 202 +202 =========================================== + Hits 0 1090 +1090 ```

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